'\" et
.TH AWK "1P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
awk
\(em pattern scanning and processing language
.SH SYNOPSIS
.LP
.nf
awk \fB[\fR-F \fIsepstring\fB] [\fR-v \fIassignment\fB]\fR... \fIprogram\fB [\fIargument\fR...\fB]\fR
.P
awk \fB[\fR-F \fIsepstring\fB] \fR-f \fIprogfile \fB[\fR-f \fIprogfile\fB]\fR... \fB[\fR-v \fIassignment\fB]\fR...
     \fB[\fIargument\fR...\fB]\fR
.fi
.SH DESCRIPTION
The
.IR awk
utility shall execute programs written in the
.IR awk
programming language, which is specialized for textual data
manipulation. An
.IR awk
program is a sequence of patterns and corresponding actions. When
input is read that matches a pattern, the action associated with that
pattern is carried out.
.P
Input shall be interpreted as a sequence of records. By default, a
record is a line, less its terminating
<newline>,
but this can be changed by using the
.BR RS
built-in variable. Each record of input shall be matched in turn
against each pattern in the program. For each pattern matched, the
associated action shall be executed.
.P
The
.IR awk
utility shall interpret each input record as a sequence of fields
where, by default, a field is a string of non-\c
<blank>
non-\c
<newline>
characters. This default
<blank>
and
<newline>
field delimiter can be changed by using the
.BR FS
built-in variable or the
.BR \-F
.IR sepstring
option. The
.IR awk
utility shall denote the first field in a record $1, the second $2, and
so on. The symbol $0 shall refer to the entire record; setting any
other field causes the re-evaluation of $0. Assigning to $0 shall reset
the values of all other fields and the
.BR NF
built-in variable.
.SH OPTIONS
The
.IR awk
utility shall conform to the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 12.2" ", " "Utility Syntax Guidelines".
.P
The following options shall be supported:
.IP "\fB\-F\ \fIsepstring\fR" 10
Define the input field separator. This option shall be equivalent to:
.RS 10 
.sp
.RS 4
.nf

-v FS=\fIsepstring
.fi
.P
.RE
.P
except that if
.BR \-F
.IR sepstring
and
.BR \-v
.IR \fRFS=\fPsepstring\fR
are both used, it is unspecified whether the
.BR FS
assignment resulting from
.BR \-F
.IR sepstring
is processed in command line order or is processed after the last
.BR \-v
.IR \fRFS=\fPsepstring\fR .
See the description of the
.BR FS
built-in variable, and how it is used, in the EXTENDED DESCRIPTION
section.
.RE
.IP "\fB\-f\ \fIprogfile\fR" 10
Specify the pathname of the file
.IR progfile
containing an
.IR awk
program. A pathname of
.BR '\-' 
shall denote the standard input. If multiple instances of this option
are specified, the concatenation of the files specified as
.IR progfile
in the order specified shall be the
.IR awk
program. The
.IR awk
program can alternatively be specified in the command line as a single
argument.
.IP "\fB\-v\ \fIassignment\fR" 10
.br
The application shall ensure that the
.IR assignment
argument is in the same form as an
.IR assignment
operand. The specified variable assignment shall occur prior to
executing the
.IR awk
program, including the actions associated with
.BR BEGIN
patterns (if any). Multiple occurrences of this option can be
specified.
.SH OPERANDS
The following operands shall be supported:
.IP "\fIprogram\fR" 10
If no
.BR \-f
option is specified, the first operand to
.IR awk
shall be the text of the
.IR awk
program. The application shall supply the
.IR program
operand as a single argument to
.IR awk .
If the text does not end in a
<newline>,
.IR awk
shall interpret the text as if it did.
.IP "\fIargument\fR" 10
Either of the following two types of
.IR argument
can be intermixed:
.RS 10 
.IP "\fIfile\fR" 10
A pathname of a file that contains the input to be read, which is
matched against the set of patterns in the program. If no
.IR file
operands are specified, or if a
.IR file
operand is
.BR '\-' ,
the standard input shall be used.
.IP "\fIassignment\fR" 10
An operand that begins with an
<underscore>
or alphabetic character from the portable character set (see the table
in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 6.1" ", " "Portable Character Set"),
followed by a sequence of underscores, digits, and alphabetics from the
portable character set, followed by the
.BR '=' 
character, shall specify a variable assignment rather than a pathname.
The characters before the
.BR '=' 
represent the name of an
.IR awk
variable; if that name is an
.IR awk
reserved word (see
.IR "Grammar")
the behavior is undefined. The characters following the
<equals-sign>
shall be interpreted as if they appeared in the
.IR awk
program preceded and followed by a double-quote (\c
.BR '\&"' )
character, as a
.BR STRING
token (see
.IR "Grammar"),
except that if the last character is an unescaped
<backslash>,
it shall be interpreted as a literal
<backslash>
rather than as the first character of the sequence
.BR \(dq\e"\(dq .
The variable shall be assigned the value of that
.BR STRING
token and, if appropriate, shall be considered a
.IR "numeric string"
(see
.IR "Expressions in awk"),
the variable shall also be assigned its numeric value. Each such
variable assignment shall occur just prior to the processing of the
following
.IR file ,
if any. Thus, an assignment before the first
.IR file
argument shall be executed after the
.BR BEGIN
actions (if any), while an assignment after the last
.IR file
argument shall occur before the
.BR END
actions (if any). If there are no
.IR file
arguments, assignments shall be executed before processing the standard
input.
.RE
.SH STDIN
The standard input shall be used only if no
.IR file
operands are specified, or if a
.IR file
operand is
.BR '\-' ,
or if a
.IR progfile
option-argument is
.BR '\-' ;
see the INPUT FILES section. If the
.IR awk
program contains no actions and no patterns, but is otherwise a valid
.IR awk
program, standard input and any
.IR file
operands shall not be read and
.IR awk
shall exit with a return status of zero.
.SH "INPUT FILES"
Input files to the
.IR awk
program from any of the following sources shall be text files:
.IP " *" 4
Any
.IR file
operands or their equivalents, achieved by modifying the
.IR awk
variables
.BR ARGV
and
.BR ARGC
.IP " *" 4
Standard input in the absence of any
.IR file
operands
.IP " *" 4
Arguments to the
.BR getline
function
.P
Whether the variable
.BR RS
is set to a value other than a
<newline>
or not, for these files, implementations shall support records
terminated with the specified separator up to
{LINE_MAX}
bytes and may support longer records.
.P
If
.BR \-f
.IR progfile
is specified, the application shall ensure that the files named by each
of the
.IR progfile
option-arguments are text files and their concatenation, in the same
order as they appear in the arguments, is an
.IR awk
program.
.SH "ENVIRONMENT VARIABLES"
The following environment variables shall affect the execution of
.IR awk :
.IP "\fILANG\fP" 10
Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 8.2" ", " "Internationalization Variables"
for the precedence of internationalization variables used to determine
the values of locale categories.)
.IP "\fILC_ALL\fP" 10
If set to a non-empty string value, override the values of all the
other internationalization variables.
.IP "\fILC_COLLATE\fP" 10
.br
Determine the locale for the behavior of ranges, equivalence classes,
and multi-character collating elements within regular expressions and
in comparisons of string values.
.IP "\fILC_CTYPE\fP" 10
Determine the locale for the interpretation of sequences of bytes of
text data as characters (for example, single-byte as opposed to
multi-byte characters in arguments and input files), the behavior of
character classes within regular expressions, the identification of
characters as letters, and the mapping of uppercase and lowercase
characters for the
.BR toupper
and
.BR tolower
functions.
.IP "\fILC_MESSAGES\fP" 10
.br
Determine the locale that should be used to affect the format and
contents of diagnostic messages written to standard error.
.IP "\fILC_NUMERIC\fP" 10
.br
Determine the radix character used when interpreting numeric input,
performing conversions between numeric and string values, and
formatting numeric output. Regardless of locale, the
<period>
character (the decimal-point character of the POSIX locale) is the
decimal-point character recognized in processing
.IR awk
programs (including assignments in command line arguments).
.IP "\fINLSPATH\fP" 10
Determine the location of message catalogs for the processing of
.IR LC_MESSAGES .
.IP "\fIPATH\fP" 10
Determine the search path when looking for commands executed by
\fIsystem\fR(\fIexpr\fR), or input and output pipes; see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 8" ", " "Environment Variables".
.P
In addition, all environment variables shall be visible via the
.IR awk
variable
.BR ENVIRON .
.SH "ASYNCHRONOUS EVENTS"
Default.
.SH STDOUT
The nature of the output files depends on the
.IR awk
program.
.SH STDERR
The standard error shall be used only for diagnostic messages.
.SH "OUTPUT FILES"
The nature of the output files depends on the
.IR awk
program.
.br
.SH "EXTENDED DESCRIPTION"
.SS "Overall Program Structure"
.P
An
.IR awk
program is composed of pairs of the form:
.sp
.RS 4
.nf

\fIpattern\fR { \fIaction\fR }
.fi
.P
.RE
.P
Either the pattern or the action (including the enclosing brace
characters) can be omitted.
.P
A missing pattern shall match any record of input, and a missing action
shall be equivalent to:
.sp
.RS 4
.nf

{ print }
.fi
.P
.RE
.P
Execution of the
.IR awk
program shall start by first executing the actions associated with all
.BR BEGIN
patterns in the order they occur in the program. Then each
.IR file
operand (or standard input if no files were specified) shall be
processed in turn by reading data from the file until a record
separator is seen (\c
<newline>
by default). Before the first reference to a field in the record is
evaluated, the record shall be split into fields, according to the
rules in
.IR "Regular Expressions",
using the value of
.BR FS
that was current at the time the record was read. Each pattern in the
program then shall be evaluated in the order of occurrence, and the
action associated with each pattern that matches the current record
executed. The action for a matching pattern shall be executed before
evaluating subsequent patterns. Finally, the actions associated with
all
.BR END
patterns shall be executed in the order they occur in the program.
.SS "Expressions in awk"
.P
Expressions describe computations used in
.IR patterns
and
.IR actions .
In the following table, valid expression operations are given in groups
from highest precedence first to lowest precedence last, with
equal-precedence operators grouped between horizontal lines. In
expression evaluation, where the grammar is formally ambiguous, higher
precedence operators shall be evaluated before lower precedence
operators. In this table
.IR expr ,
.IR expr1 ,
.IR expr2 ,
and
.IR expr3
represent any expression, while lvalue represents any entity that can
be assigned to (that is, on the left side of an assignment operator).
The precise syntax of expressions is given in
.IR "Grammar".
.sp
.ce 1
\fBTable 4-1: Expressions in Decreasing Precedence in \fIawk\fP\fR
.TS
box tab(@) center;
cB | cB | cB | cB
l1f5 | l1 | l1 | l.
Syntax@Name@Type of Result@Associativity
_
( \fIexpr\fP )@Grouping@Type of \fIexpr\fP@N/A
_
$\fIexpr\fP@Field reference@String@N/A
_
lvalue ++@Post-increment@Numeric@N/A
lvalue \-\|\-@Post-decrement@Numeric@N/A
_
++ lvalue@Pre-increment@Numeric@N/A
\-\|\- lvalue@Pre-decrement@Numeric@N/A
_
\fIexpr\fP ^ \fIexpr\fP@Exponentiation@Numeric@Right
_
! \fIexpr\fP@Logical not@Numeric@N/A
+ \fIexpr\fP@Unary plus@Numeric@N/A
\- \fIexpr\fP@Unary minus@Numeric@N/A
_
\fIexpr\fP * \fIexpr\fP@Multiplication@Numeric@Left
\fIexpr\fP / \fIexpr\fP@Division@Numeric@Left
\fIexpr\fP % \fIexpr\fP@Modulus@Numeric@Left
_
\fIexpr\fP + \fIexpr\fP@Addition@Numeric@Left
\fIexpr\fP \- \fIexpr\fP@Subtraction@Numeric@Left
_
\fIexpr\fP \fIexpr\fP@String concatenation@String@Left
_
\fIexpr\fP < \fIexpr\fP@Less than@Numeric@None
\fIexpr\fP <= \fIexpr\fP@Less than or equal to@Numeric@None
\fIexpr\fP != \fIexpr\fP@Not equal to@Numeric@None
\fIexpr\fP == \fIexpr\fP@Equal to@Numeric@None
\fIexpr\fP > \fIexpr\fP@Greater than@Numeric@None
\fIexpr\fP >= \fIexpr\fP@Greater than or equal to@Numeric@None
_
\fIexpr\fP ~ \fIexpr\fP@ERE match@Numeric@None
\fIexpr\fP !~ \fIexpr\fP@ERE non-match@Numeric@None
_
\fIexpr\fP in array@Array membership@Numeric@Left
( \fIindex\fP ) in \fIarray\fP@Multi-dimension array@Numeric@Left
@membership
_
\fIexpr\fP && \fIexpr\fP@Logical AND@Numeric@Left
_
\fIexpr\fP || \fIexpr\fP@Logical OR@Numeric@Left
_
\fIexpr1\fP ? \fIexpr2\fP : \fIexpr3\fP@Conditional expression@Type of selected@Right
@@\fIexpr2\fP or \fIexpr3\fP
_
lvalue ^= \fIexpr\fP@Exponentiation assignment@Numeric@Right
lvalue %= \fIexpr\fP@Modulus assignment@Numeric@Right
lvalue *= \fIexpr\fP@Multiplication assignment@Numeric@Right
lvalue /= \fIexpr\fP@Division assignment@Numeric@Right
lvalue += \fIexpr\fP@Addition assignment@Numeric@Right
lvalue \-= \fIexpr\fP@Subtraction assignment@Numeric@Right
lvalue = \fIexpr\fP@Assignment@Type of \fIexpr\fP@Right
.TE
.P
Each expression shall have either a string value, a numeric value, or
both. Except as stated for specific contexts, the value of an expression
shall be implicitly converted to the type needed for the context in which
it is used. A string value shall be converted to a numeric value either by
the equivalent of the following calls to functions defined by the ISO\ C standard:
.sp
.RS 4
.nf

setlocale(LC_NUMERIC, "");
\fInumeric_value\fR = atof(\fIstring_value\fR);
.fi
.P
.RE
.P
or by converting the initial portion of the string to type
.BR double
representation as follows:
.sp
.RS
The input string is decomposed into two parts: an initial, possibly empty,
sequence of white-space characters (as specified by
\fIisspace\fR())
and a subject sequence interpreted as a floating-point constant.
.P
The expected form of the subject sequence is an optional
.BR '+' 
or
.BR '\-' 
sign, then a non-empty sequence of digits optionally containing a
<period>,
then an optional exponent part. An exponent part consists of
.BR 'e' 
or
.BR 'E' ,
followed by an optional sign, followed by one or more decimal digits.
.P
The sequence starting with the first digit or the
<period>
(whichever occurs first) is interpreted as a floating constant of the
C language, and if neither an exponent part nor a
<period>
appears, a
<period>
is assumed to follow the last digit in the string. If the subject
sequence begins with a
<hyphen-minus>,
the value resulting from the conversion is negated.
.RE
.P
A numeric value that is exactly equal to the value of an integer (see
.IR "Section 1.1.2" ", " "Concepts Derived from the ISO C Standard")
shall be converted to a string by the equivalent of a call to the
.BR sprintf
function (see
.IR "String Functions")
with the string
.BR \(dq%d\(dq 
as the
.IR fmt
argument and the numeric value being converted as the first and only
.IR expr
argument. Any other numeric value shall be converted to a string by the
equivalent of a call to the
.BR sprintf
function with the value of the variable
.BR CONVFMT
as the
.IR fmt
argument and the numeric value being converted as the first and only
.IR expr
argument. The result of the conversion is unspecified if the value of
.BR CONVFMT
is not a floating-point format specification. This volume of POSIX.1\(hy2017 specifies no
explicit conversions between numbers and strings. An application can
force an expression to be treated as a number by adding zero to it, or
can force it to be treated as a string by concatenating the null string
(\c
.BR \(dq\^\(dq )
to it.
.P
A string value shall be considered a
.IR "numeric string"
if it comes from one of the following:
.IP " 1." 4
Field variables
.IP " 2." 4
Input from the
\fIgetline\fR()
function
.IP " 3." 4
.BR FILENAME
.IP " 4." 4
.BR ARGV
array elements
.IP " 5." 4
.BR ENVIRON
array elements
.IP " 6." 4
Array elements created by the
\fIsplit\fR()
function
.IP " 7." 4
A command line variable assignment
.IP " 8." 4
Variable assignment from another numeric string variable
.P
and an implementation-dependent condition corresponding to either
case (a) or (b) below is met.
.IP " a." 4
After the equivalent of the following calls to functions defined by
the ISO\ C standard,
.IR string_value_end
would differ from
.IR string_value ,
and any characters before the terminating null character in
.IR string_value_end
would be
<blank>
characters:
.RS 4 
.sp
.RS 4
.nf

char *string_value_end;
setlocale(LC_NUMERIC, "");
numeric_value = strtod (string_value, &string_value_end);
.fi
.P
.RE
.RE
.IP " b." 4
After all the following conversions have been applied, the resulting
string would lexically be recognized as a
.BR NUMBER
token as described by the lexical conventions in
.IR "Grammar":
.RS 4 
.IP -- 4
All leading and trailing
<blank>
characters are discarded.
.IP -- 4
If the first non-\c
<blank>
is
.BR '\(pl' 
or
.BR '\-' ,
it is discarded.
.IP -- 4
Each occurrence of the decimal point character from the current locale
is changed to a
<period>.
.RE
In case (a) the numeric value of the
.IR "numeric string"
shall be the value that would be returned by the
\fIstrtod\fR()
call. In case (b) if the first non-\c
<blank>
is
.BR '\-' ,
the numeric value of the
.IR "numeric string"
shall be the negation of the numeric value of the recognized
.BR NUMBER
token; otherwise, the numeric value of the
.IR "numeric string"
shall be the numeric value of the recognized
.BR NUMBER
token. Whether or not a string is a
.IR "numeric string"
shall be relevant only in contexts where that term is used in this
section.
.P
When an expression is used in a Boolean context, if it has a numeric
value, a value of zero shall be treated as false and any other value
shall be treated as true. Otherwise, a string value of the null string
shall be treated as false and any other value shall be treated as true.
A Boolean context shall be one of the following:
.IP " *" 4
The first subexpression of a conditional expression
.IP " *" 4
An expression operated on by logical NOT, logical AND, or logical OR
.IP " *" 4
The second expression of a
.BR for
statement
.IP " *" 4
The expression of an
.BR if
statement
.IP " *" 4
The expression of the
.BR while
clause in either a
.BR while
or
.BR do .\|.\|.\c
.BR while
statement
.IP " *" 4
An expression used as a pattern (as in Overall Program Structure)
.P
All arithmetic shall follow the semantics of floating-point arithmetic as
specified by the ISO\ C standard (see
.IR "Section 1.1.2" ", " "Concepts Derived from the ISO C Standard").
.P
The value of the expression:
.sp
.RS 4
.nf

\fIexpr1\fR \(ha \fIexpr2\fR
.fi
.P
.RE
.P
shall be equivalent to the value returned by the ISO\ C standard function call:
.sp
.RS 4
.nf

\fRpow(\fIexpr1\fR, \fIexpr2\fR)
.fi
.P
.RE
.P
The expression:
.sp
.RS 4
.nf

lvalue \(ha= \fIexpr\fR
.fi
.P
.RE
.P
shall be equivalent to the ISO\ C standard expression:
.sp
.RS 4
.nf

lvalue = pow(lvalue, \fIexpr\fR)
.fi
.P
.RE
.P
except that lvalue shall be evaluated only once. The value of the
expression:
.sp
.RS 4
.nf

\fIexpr1\fR % \fIexpr2\fR
.fi
.P
.RE
.P
shall be equivalent to the value returned by the ISO\ C standard function call:
.sp
.RS 4
.nf

fmod(\fIexpr1\fR, \fIexpr2\fR)
.fi
.P
.RE
.P
The expression:
.sp
.RS 4
.nf

lvalue %= \fIexpr\fR
.fi
.P
.RE
.P
shall be equivalent to the ISO\ C standard expression:
.sp
.RS 4
.nf

lvalue = fmod(lvalue, \fIexpr\fR)
.fi
.P
.RE
.P
except that lvalue shall be evaluated only once.
.P
Variables and fields shall be set by the assignment statement:
.sp
.RS 4
.nf

lvalue = \fIexpression\fR
.fi
.P
.RE
.P
and the type of
.IR expression
shall determine the resulting variable type. The assignment includes
the arithmetic assignments (\c
.BR \(dq+=\(dq ,
.BR \(dq-=\(dq ,
.BR \(dq*=\(dq ,
.BR \(dq/=\(dq ,
.BR \(dq%=\(dq ,
.BR \(dq\(ha=\(dq ,
.BR \(dq++\(dq ,
.BR \(dq--\(dq )
all of which shall produce a numeric result. The left-hand side of an
assignment and the target of increment and decrement operators can be
one of a variable, an array with index, or a field selector.
.P
The
.IR awk
language supplies arrays that are used for storing numbers or strings.
Arrays need not be declared. They shall initially be empty, and their
sizes shall change dynamically. The subscripts, or element identifiers,
are strings, providing a type of associative array capability. An array
name followed by a subscript within square brackets can be used as an
lvalue and thus as an expression, as described in the grammar; see
.IR "Grammar".
Unsubscripted array names can be used in only the following contexts:
.IP " *" 4
A parameter in a function definition or function call
.IP " *" 4
The
.BR NAME
token following any use of the keyword
.BR in
as specified in the grammar (see
.IR "Grammar");
if the name used in this context is not an array name, the behavior is
undefined
.P
A valid array
.IR index
shall consist of one or more
<comma>-separated
expressions, similar to the way in which multi-dimensional arrays are
indexed in some programming languages. Because
.IR awk
arrays are really one-dimensional, such a
<comma>-separated
list shall be converted to a single string by concatenating the string
values of the separate expressions, each separated from the other by
the value of the
.BR SUBSEP
variable. Thus, the following two index operations shall be
equivalent:
.sp
.RS 4
.nf

\fIvar\fB[\fIexpr1\fR, \fIexpr2\fR, ... \fIexprn\fB]
.P
\fIvar\fB[\fIexpr1\fR SUBSEP \fIexpr2\fR SUBSEP ... \fRSUBSEP \fIexprn\fB]\fR
.fi
.P
.RE
.P
The application shall ensure that a multi-dimensioned
.IR index
used with the
.BR in
operator is parenthesized. The
.BR in
operator, which tests for the existence of a particular array element,
shall not cause that element to exist. Any other reference to a
nonexistent array element shall automatically create it.
.P
Comparisons (with the
.BR '<' ,
.BR \(dq<=\(dq ,
.BR \(dq!=\(dq ,
.BR \(dq==\(dq ,
.BR '>' ,
and
.BR \(dq>=\(dq 
operators) shall be made numerically if both operands are numeric, if
one is numeric and the other has a string value that is a numeric
string, or if one is numeric and the other has the uninitialized value.
Otherwise, operands shall be converted to strings as required and a
string comparison shall be made as follows:
.IP " *" 4
For the
.BR \(dq!=\(dq 
and
.BR \(dq==\(dq 
operators, the strings should be compared to check if they are
identical but may be compared using the locale-specific collation
sequence to check if they collate equally.
.IP " *" 4
For the other operators, the strings shall be compared using the
locale-specific collation sequence.
.P
The value of the comparison expression shall be 1 if the relation is
true, or 0 if the relation is false.
.SS "Variables and Special Variables"
.P
Variables can be used in an
.IR awk
program by referencing them. With the exception of function parameters
(see
.IR "User-Defined Functions"),
they are not explicitly declared. Function parameter names shall be
local to the function; all other variable names shall be global. The
same name shall not be used as both a function parameter name and as
the name of a function or a special
.IR awk
variable. The same name shall not be used both as a variable name with
global scope and as the name of a function. The same name shall not be
used within the same scope both as a scalar variable and as an array.
Uninitialized variables, including scalar variables, array elements,
and field variables, shall have an uninitialized value. An
uninitialized value shall have both a numeric value of zero and a
string value of the empty string. Evaluation of variables with an
uninitialized value, to either string or numeric, shall be determined
by the context in which they are used.
.P
Field variables shall be designated by a
.BR '$' 
followed by a number or numerical expression. The effect of the field
number
.IR expression
evaluating to anything other than a non-negative integer is
unspecified; uninitialized variables or string values need not be
converted to numeric values in this context. New field variables can be
created by assigning a value to them. References to nonexistent fields
(that is, fields after $\fBNF\fP), shall evaluate to the uninitialized
value. Such references shall not create new fields. However, assigning
to a nonexistent field (for example, $(\fBNF\fP+2)=5) shall increase
the value of
.BR NF ;
create any intervening fields with the uninitialized value; and cause
the value of $0 to be recomputed, with the fields being separated by
the value of
.BR OFS .
Each field variable shall have a string value or an uninitialized value
when created. Field variables shall have the uninitialized value when
created from $0 using
.BR FS
and the variable does not contain any characters. If appropriate, the
field variable shall be considered a numeric string (see
.IR "Expressions in awk").
.P
Implementations shall support the following other special variables
that are set by
.IR awk :
.IP "\fBARGC\fR" 10
The number of elements in the
.BR ARGV
array.
.IP "\fBARGV\fR" 10
An array of command line arguments, excluding options and the
.IR program
argument, numbered from zero to
.BR ARGC \-1.
.RS 10 
.P
The arguments in
.BR ARGV
can be modified or added to;
.BR ARGC
can be altered. As each input file ends,
.IR awk
shall treat the next non-null element of
.BR ARGV ,
up to the current value of
.BR ARGC \-1,
inclusive, as the name of the next input file. Thus, setting an element
of
.BR ARGV
to null means that it shall not be treated as an input file. The name
.BR '\-' 
indicates the standard input. If an argument matches the format of an
.IR assignment
operand, this argument shall be treated as an
.IR assignment
rather than a
.IR file
argument.
.RE
.IP "\fBCONVFMT\fR" 10
The
.BR printf
format for converting numbers to strings (except for output statements,
where
.BR OFMT
is used);
.BR \(dq%.6g\(dq 
by default.
.IP "\fBENVIRON\fR" 10
An array representing the value of the environment, as described in the
.IR exec
functions defined in the System Interfaces volume of POSIX.1\(hy2017. The indices of the array shall be
strings consisting of the names of the environment variables, and the
value of each array element shall be a string consisting of the value
of that variable. If appropriate, the environment variable shall be
considered a
.IR "numeric string"
(see
.IR "Expressions in awk");
the array element shall also have its numeric value.
.RS 10 
.P
In all cases where the behavior of
.IR awk
is affected by environment variables (including the environment of any
commands that
.IR awk
executes via the
.BR system
function or via pipeline redirections with the
.BR print
statement, the
.BR printf
statement, or the
.BR getline
function), the environment used shall be the environment at the time
.IR awk
began executing; it is implementation-defined whether any
modification of
.BR ENVIRON
affects this environment.
.RE
.IP "\fBFILENAME\fR" 10
A pathname of the current input file. Inside a
.BR BEGIN
action the value is undefined. Inside an
.BR END
action the value shall be the name of the last input file processed.
.IP "\fBFNR\fR" 10
The ordinal number of the current record in the current file. Inside a
.BR BEGIN
action the value shall be zero. Inside an
.BR END
action the value shall be the number of the last record processed in
the last file processed.
.IP "\fBFS\fR" 10
Input field separator regular expression; a
<space>
by default.
.IP "\fBNF\fR" 10
The number of fields in the current record. Inside a
.BR BEGIN
action, the use of
.BR NF
is undefined unless a
.BR getline
function without a
.IR var
argument is executed previously. Inside an
.BR END
action,
.BR NF
shall retain the value it had for the last record read, unless a
subsequent, redirected,
.BR getline
function without a
.IR var
argument is performed prior to entering the
.BR END
action.
.IP "\fBNR\fR" 10
The ordinal number of the current record from the start of input.
Inside a
.BR BEGIN
action the value shall be zero. Inside an
.BR END
action the value shall be the number of the last record processed.
.IP "\fBOFMT\fR" 10
The
.BR printf
format for converting numbers to strings in output statements (see
.IR "Output Statements");
.BR \(dq%.6g\(dq 
by default. The result of the conversion is unspecified if the value of
.BR OFMT
is not a floating-point format specification.
.IP "\fBOFS\fR" 10
The
.BR print
statement output field separator;
<space>
by default.
.IP "\fBORS\fR" 10
The
.BR print
statement output record separator; a
<newline>
by default.
.IP "\fBRLENGTH\fR" 10
The length of the string matched by the
.BR match
function.
.IP "\fBRS\fR" 10
The first character of the string value of
.BR RS
shall be the input record separator; a
<newline>
by default. If
.BR RS
contains more than one character, the results are unspecified. If
.BR RS
is null, then records are separated by sequences consisting of a
<newline>
plus one or more blank lines, leading or trailing blank lines shall not
result in empty records at the beginning or end of the input, and a
<newline>
shall always be a field separator, no matter what the value of
.BR FS
is.
.IP "\fBRSTART\fR" 10
The starting position of the string matched by the
.BR match
function, numbering from 1. This shall always be equivalent to the
return value of the
.BR match
function.
.IP "\fBSUBSEP\fR" 10
The subscript separator string for multi-dimensional arrays; the
default value is implementation-defined.
.SS "Regular Expressions"
.P
The
.IR awk
utility shall make use of the extended regular expression notation
(see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 9.4" ", " "Extended Regular Expressions")
except that it shall allow the use of C-language conventions
for escaping special characters within the EREs, as specified in the
table in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 5" ", " "File Format Notation"
(\c
.BR '\e\e' ,
.BR '\ea' ,
.BR '\eb' ,
.BR '\ef' ,
.BR '\en' ,
.BR '\er' ,
.BR '\et' ,
.BR '\ev' )
and the following table; these escape sequences shall be recognized
both inside and outside bracket expressions. Note that records need not
be separated by
<newline>
characters and string constants can contain
<newline>
characters, so even the
.BR \(dq\en\(dq 
sequence is valid in
.IR awk
EREs. Using a
<slash>
character within an ERE requires the escaping shown in the following
table.
.br
.sp
.ce 1
\fBTable 4-2: Escape Sequences in \fIawk\fP\fR
.ad l
.TS
center tab(@) box;
cB | cB | cB
cB | cB | cB
lf5 | lw(34) | lw(34).
Escape
Sequence@Description@Meaning
_
\e"@T{
<backslash> <quotation-mark>
T}@T{
<quotation-mark> character
T}
_
\e/@T{
<backslash> <slash>
T}@T{
<slash> character
T}
_
\eddd@T{
A
<backslash>
character followed by the longest sequence of one, two, or
three octal-digit characters (01234567). If all of the digits are 0
(that is, representation of the NUL character), the behavior is
undefined.
T}@T{
The character whose encoding is represented by the one, two, or
three-digit octal integer. Multi-byte characters require
multiple, concatenated escape sequences of this type, including the
leading
<backslash>
for each byte.
T}
_
\ec@T{
A
<backslash>
character followed by any character not described in this
table or in the table in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 5" ", " "File Format Notation"
(\c
.BR '\e\e' ,
.BR '\ea' ,
.BR '\eb' ,
.BR '\ef' ,
.BR '\en' ,
.BR '\er' ,
.BR '\et' ,
.BR '\ev' ).
T}@Undefined
.TE
.ad b
.P
A regular expression can be matched against a specific field or string
by using one of the two regular expression matching operators,
.BR '\(ti' 
and
.BR \(dq!\(ti\(dq .
These operators shall interpret their right-hand operand as a regular
expression and their left-hand operand as a string. If the regular
expression matches the string, the
.BR '\(ti' 
expression shall evaluate to a value of 1, and the
.BR \(dq!\(ti\(dq 
expression shall evaluate to a value of 0. (The regular expression
matching operation is as defined by the term matched in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 9.1" ", " "Regular Expression Definitions",
where a match occurs on any part of the string unless the regular
expression is limited with the
<circumflex>
or
<dollar-sign>
special characters.) If the regular expression does not match the
string, the
.BR '\(ti' 
expression shall evaluate to a value of 0, and the
.BR \(dq!\(ti\(dq 
expression shall evaluate to a value of 1. If the right-hand operand is
any expression other than the lexical token
.BR ERE ,
the string value of the expression shall be interpreted as an extended
regular expression, including the escape conventions described above.
Note that these same escape conventions shall also be applied in
determining the value of a string literal (the lexical token
.BR STRING ),
and thus shall be applied a second time when a string literal is used
in this context.
.P
When an
.BR ERE
token appears as an expression in any context other than as the
right-hand of the
.BR '\(ti' 
or
.BR \(dq!\(ti\(dq 
operator or as one of the built-in function arguments described below,
the value of the resulting expression shall be the equivalent of:
.sp
.RS 4
.nf

$0 \(ti /\fIere\fR/
.fi
.P
.RE
.P
The
.IR ere
argument to the
.BR gsub ,
.BR match ,
.BR sub
functions, and the
.IR fs
argument to the
.BR split
function (see
.IR "String Functions")
shall be interpreted as extended regular expressions. These can be
either
.BR ERE
tokens or arbitrary expressions, and shall be interpreted in the same
manner as the right-hand side of the
.BR '\(ti' 
or
.BR \(dq!\(ti\(dq 
operator.
.P
An extended regular expression can be used to separate fields by assigning
a string containing the expression to the built-in variable
.BR FS ,
either directly or as a consequence of using the
.BR \-F
.IR sepstring
option.
The default value of the
.BR FS
variable shall be a single
<space>.
The following describes
.BR FS
behavior:
.IP " 1." 4
If
.BR FS
is a null string, the behavior is unspecified.
.IP " 2." 4
If
.BR FS
is a single character:
.RS 4 
.IP " a." 4
If
.BR FS
is
<space>,
skip leading and trailing
<blank>
and
<newline>
characters; fields shall be delimited by sets of one or more
<blank>
or
<newline>
characters.
.IP " b." 4
Otherwise, if
.BR FS
is any other character
.IR c ,
fields shall be delimited by each single occurrence of
.IR c .
.RE
.IP " 3." 4
Otherwise, the string value of
.BR FS
shall be considered to be an extended regular expression. Each
occurrence of a sequence matching the extended regular expression shall
delimit fields.
.P
Except for the
.BR '\(ti' 
and
.BR \(dq!\(ti\(dq 
operators, and in the
.BR gsub ,
.BR match ,
.BR split ,
and
.BR sub
built-in functions, ERE matching shall be based on input records; that
is, record separator characters (the first character of the value of
the variable
.BR RS ,
<newline>
by default) cannot be embedded in the expression, and no expression
shall match the record separator character. If the record separator is
not
<newline>,
<newline>
characters embedded in the expression can be matched. For the
.BR '\(ti' 
and
.BR \(dq!\(ti\(dq 
operators, and in those four built-in functions, ERE matching shall be
based on text strings; that is, any character (including
<newline>
and the record separator) can be embedded in the pattern, and an
appropriate pattern shall match any character. However, in all
.IR awk
ERE matching, the use of one or more NUL characters in the pattern,
input record, or text string produces undefined results.
.SS "Patterns"
.P
A
.IR pattern
is any valid
.IR expression ,
a range specified by two expressions separated by a comma, or one of the
two special patterns
.BR BEGIN
or
.BR END .
.SS "Special Patterns"
.P
The
.IR awk
utility shall recognize two special patterns,
.BR BEGIN
and
.BR END .
Each
.BR BEGIN
pattern shall be matched once and its associated action executed before
the first record of input is read\(emexcept possibly by use of the
.BR getline
function (see
.IR "Input/Output and General Functions")
in a prior
.BR BEGIN
action\(emand before command line assignment is done. Each
.BR END
pattern shall be matched once and its associated action executed after
the last record of input has been read. These two patterns shall have
associated actions.
.P
.BR BEGIN
and
.BR END
shall not combine with other patterns. Multiple
.BR BEGIN
and
.BR END
patterns shall be allowed. The actions associated with the
.BR BEGIN
patterns shall be executed in the order specified in the program, as
are the
.BR END
actions. An
.BR END
pattern can precede a
.BR BEGIN
pattern in a program.
.P
If an
.IR awk
program consists of only actions with the pattern
.BR BEGIN ,
and the
.BR BEGIN
action contains no
.BR getline
function,
.IR awk
shall exit without reading its input when the last statement in the
last
.BR BEGIN
action is executed. If an
.IR awk
program consists of only actions with the pattern
.BR END
or only actions with the patterns
.BR BEGIN
and
.BR END ,
the input shall be read before the statements in the
.BR END
actions are executed.
.SS "Expression Patterns"
.P
An expression pattern shall be evaluated as if it were an expression in
a Boolean context. If the result is true, the pattern shall be
considered to match, and the associated action (if any) shall be
executed. If the result is false, the action shall not be executed.
.SS "Pattern Ranges"
.P
A pattern range consists of two expressions separated by a comma; in
this case, the action shall be performed for all records between a
match of the first expression and the following match of the second
expression, inclusive. At this point, the pattern range can be repeated
starting at input records subsequent to the end of the matched range.
.SS "Actions"
.P
An action is a sequence of statements as shown in the grammar in
.IR "Grammar".
Any single statement can be replaced by a statement list enclosed in
curly braces. The application shall ensure that statements in a
statement list are separated by
<newline>
or
<semicolon>
characters. Statements in a statement list shall be executed sequentially
in the order that they appear.
.P
The
.IR expression
acting as the conditional in an
.BR if
statement shall be evaluated and if it is non-zero or non-null, the
following statement shall be executed; otherwise, if
.BR else
is present, the statement following the
.BR else
shall be executed.
.P
The
.BR if ,
.BR while ,
.BR do .\|.\|.\c
.BR while ,
.BR for ,
.BR break ,
and
.BR continue
statements are based on the ISO\ C standard (see
.IR "Section 1.1.2" ", " "Concepts Derived from the ISO C Standard"),
except that the Boolean expressions shall be treated as described in
.IR "Expressions in awk",
and except in the case of:
.sp
.RS 4
.nf

for (\fIvariable\fR in \fIarray\fR)
.fi
.P
.RE
.P
which shall iterate, assigning each
.IR index
of
.IR array
to
.IR variable
in an unspecified order. The results of adding new elements to
.IR array
within such a
.BR for
loop are undefined. If a
.BR break
or
.BR continue
statement occurs outside of a loop, the behavior is undefined.
.P
The
.BR delete
statement shall remove an individual array element. Thus, the following
code deletes an entire array:
.sp
.RS 4
.nf

for (index in array)
    delete array[index]
.fi
.P
.RE
.P
The
.BR next
statement shall cause all further processing of the current input
record to be abandoned. The behavior is undefined if a
.BR next
statement appears or is invoked in a
.BR BEGIN
or
.BR END
action.
.P
The
.BR exit
statement shall invoke all
.BR END
actions in the order in which they occur in the program source and then
terminate the program without reading further input. An
.BR exit
statement inside an
.BR END
action shall terminate the program without further execution of
.BR END
actions. If an expression is specified in an
.BR exit
statement, its numeric value shall be the exit status of
.IR awk ,
unless subsequent errors are encountered or a subsequent
.BR exit
statement with an expression is executed.
.SS "Output Statements"
.P
Both
.BR print
and
.BR printf
statements shall write to standard output by default. The output shall
be written to the location specified by
.IR output_redirection
if one is supplied, as follows:
.sp
.RS 4
.nf

> \fIexpression\fR
>> \fIexpression\fR
| \fIexpression\fR
.fi
.P
.RE
.P
In all cases, the
.IR expression
shall be evaluated to produce a string that is used as a pathname
into which to write (for
.BR '>' 
or
.BR \(dq>>\(dq )
or as a command to be executed (for
.BR '|' ).
Using the first two forms, if the file of that name is not currently
open, it shall be opened, creating it if necessary and using the first
form, truncating the file. The output then shall be appended to the
file. As long as the file remains open, subsequent calls in which
.IR expression
evaluates to the same string value shall simply append output to the
file. The file remains open until the
.BR close
function (see
.IR "Input/Output and General Functions")
is called with an expression that evaluates to the same string value.
.P
The third form shall write output onto a stream piped to the input of a
command. The stream shall be created if no stream is currently open
with the value of
.IR expression
as its command name. The stream created shall be equivalent to one
created by a call to the
\fIpopen\fR()
function defined in the System Interfaces volume of POSIX.1\(hy2017 with the value of
.IR expression
as the
.IR command
argument and a value of
.IR w
as the
.IR mode
argument. As long as the stream remains open, subsequent calls in which
.IR expression
evaluates to the same string value shall write output to the existing
stream. The stream shall remain open until the
.BR close
function (see
.IR "Input/Output and General Functions")
is called with an expression that evaluates to the same string value.
At that time, the stream shall be closed as if by a call to the
\fIpclose\fR()
function defined in the System Interfaces volume of POSIX.1\(hy2017.
.P
As described in detail by the grammar in
.IR "Grammar",
these output statements shall take a
<comma>-separated
list of
.IR expression s
referred to in the grammar by the non-terminal symbols
.BR expr_list ,
.BR print_expr_list ,
or
.BR print_expr_list_opt .
This list is referred to here as the
.IR "expression list" ,
and each member is referred to as an
.IR "expression argument" .
.P
The
.BR print
statement shall write the value of each expression argument onto the
indicated output stream separated by the current output field separator
(see variable
.BR OFS
above), and terminated by the output record separator (see variable
.BR ORS
above). All expression arguments shall be taken as strings, being
converted if necessary; this conversion shall be as described in
.IR "Expressions in awk",
with the exception that the
.BR printf
format in
.BR OFMT
shall be used instead of the value in
.BR CONVFMT .
An empty expression list shall stand for the whole input record ($0).
.P
The
.BR printf
statement shall produce output based on a notation similar to the
File Format Notation used to describe file formats in this volume of POSIX.1\(hy2017 (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 5" ", " "File Format Notation").
Output shall be produced as specified with the first
.IR expression
argument as the string
.IR format
and subsequent
.IR expression
arguments as the strings
.IR arg1
to
.IR argn ,
inclusive, with the following exceptions:
.IP " 1." 4
The
.IR format
shall be an actual character string rather than a graphical
representation. Therefore, it cannot contain empty character
positions. The
<space>
in the
.IR format
string, in any context other than a
.IR flag
of a conversion specification, shall be treated as an ordinary
character that is copied to the output.
.IP " 2." 4
If the character set contains a
.BR ' ' 
character and that character appears in the
.IR format
string, it shall be treated as an ordinary character that is copied to
the output.
.IP " 3." 4
The
.IR "escape sequences"
beginning with a
<backslash>
character shall be treated as sequences of ordinary characters that are
copied to the output. Note that these same sequences shall be interpreted
lexically by
.IR awk
when they appear in literal strings, but they shall not be treated
specially by the
.BR printf
statement.
.IP " 4." 4
A
.IR "field width"
or
.IR precision
can be specified as the
.BR '*' 
character instead of a digit string. In this case the next argument
from the expression list shall be fetched and its numeric value taken
as the field width or precision.
.IP " 5." 4
The implementation shall not precede or follow output from the
.BR d
or
.BR u
conversion specifier characters with
<blank>
characters not specified by the
.IR format
string.
.IP " 6." 4
The implementation shall not precede output from the
.BR o
conversion specifier character with leading zeros not specified by the
.IR format
string.
.IP " 7." 4
For the
.BR c
conversion specifier character: if the argument has a numeric value, the
character whose encoding is that value shall be output. If the value is
zero or is not the encoding of any character in the character set, the
behavior is undefined. If the argument does not have a numeric value,
the first character of the string value shall be output; if the string
does not contain any characters, the behavior is undefined.
.IP " 8." 4
For each conversion specification that consumes an argument, the next
expression argument shall be evaluated. With the exception of the
.BR c
conversion specifier character, the value shall be converted (according
to the rules specified in
.IR "Expressions in awk")
to the appropriate type for the conversion specification.
.IP " 9." 4
If there are insufficient expression arguments to satisfy all the
conversion specifications in the
.IR format
string, the behavior is undefined.
.IP 10. 4
If any character sequence in the
.IR format
string begins with a
.BR '%' 
character, but does not form a valid conversion specification, the
behavior is unspecified.
.P
Both
.BR print
and
.BR printf
can output at least
{LINE_MAX}
bytes.
.SS "Functions"
.P
The
.IR awk
language has a variety of built-in functions: arithmetic, string,
input/output, and general.
.SS "Arithmetic Functions"
.P
The arithmetic functions, except for
.BR int ,
shall be based on the ISO\ C standard (see
.IR "Section 1.1.2" ", " "Concepts Derived from the ISO C Standard").
The behavior is undefined in cases where the ISO\ C standard specifies that an
error be returned or that the behavior is undefined. Although the
grammar (see
.IR "Grammar")
permits built-in functions to appear with no arguments or parentheses,
unless the argument or parentheses are indicated as optional in the
following list (by displaying them within the
.BR \(dq[]\(dq 
brackets), such use is undefined.
.IP "\fBatan2\fR(\fIy\fR,\fIx\fR)" 10
Return arctangent of \fIy\fP/\fIx\fR in radians in the range
[\-\(*p,\(*p].
.IP "\fBcos\fR(\fIx\fR)" 10
Return cosine of \fIx\fP, where \fIx\fP is in radians.
.IP "\fBsin\fR(\fIx\fR)" 10
Return sine of \fIx\fP, where \fIx\fP is in radians.
.IP "\fBexp\fR(\fIx\fR)" 10
Return the exponential function of \fIx\fP.
.IP "\fBlog\fR(\fIx\fR)" 10
Return the natural logarithm of \fIx\fP.
.IP "\fBsqrt\fR(\fIx\fR)" 10
Return the square root of \fIx\fP.
.IP "\fBint\fR(\fIx\fR)" 10
Return the argument truncated to an integer. Truncation shall
be toward 0 when \fIx\fP>0.
.IP "\fBrand\fP(\|)" 10
Return a random number \fIn\fP, such that 0\(<=\fIn\fP<1.
.IP "\fBsrand\fR(\fB[\fIexpr\fB]\fR)" 10
Set the seed value for
.IR rand
to
.IR expr
or use the time of day if
.IR expr
is omitted. The previous seed value shall be returned.
.SS "String Functions"
.P
The string functions in the following list shall be supported.
Although the grammar (see
.IR "Grammar")
permits built-in functions to appear with no arguments or parentheses,
unless the argument or parentheses are indicated as optional in the
following list (by displaying them within the
.BR \(dq[]\(dq 
brackets), such use is undefined.
.IP "\fBgsub\fR(\fIere\fR,\ \fIrepl\fB[\fR,\ \fIin\fB]\fR)" 10
.br
Behave like
.BR sub
(see below), except that it shall replace all occurrences of the
regular expression (like the
.IR ed
utility global substitute) in $0 or in the
.IR in
argument, when specified.
.IP "\fBindex\fR(\fIs\fR,\ \fIt\fR)" 10
Return the position, in characters, numbering from 1, in string
.IR s
where string
.IR t
first occurs, or zero if it does not occur at all.
.IP "\fBlength[\fR(\fB[\fIs\fB]\fR)\fB]\fR" 10
Return the length, in characters, of its argument taken as a string, or
of the whole record, $0, if there is no argument.
.IP "\fBmatch\fR(\fIs\fR,\ \fIere\fR)" 10
Return the position, in characters, numbering from 1, in string
.IR s
where the extended regular expression
.IR ere
occurs, or zero if it does not occur at all. RSTART shall be set to the
starting position (which is the same as the returned value), zero if no
match is found; RLENGTH shall be set to the length of the matched
string, \-1 if no match is found.
.IP "\fBsplit\fR(\fIs\fR,\ \fIa\fB[\fR,\ \fIfs\ \fB]\fR)" 10
.br
Split the string
.IR s
into array elements
.IR a [1],
.IR a [2],
\&.\|.\|.,
.IR a [ n ],
and return
.IR n .
All elements of the array shall be deleted before the split is
performed. The separation shall be done with the ERE
.IR fs
or with the field separator
.BR FS
if
.IR fs
is not given. Each array element shall have a string value when created
and, if appropriate, the array element shall be considered a numeric
string (see
.IR "Expressions in awk").
The effect of a null string as the value of
.IR fs
is unspecified.
.IP "\fBsprintf\fR(\fIfmt\fR,\ \fIexpr\fR,\ \fIexpr\fR,\ .\|.\|.)" 10
.br
Format the expressions according to the
.BR printf
format given by
.IR fmt
and return the resulting string.
.IP "\fBsub(\fIere\fR,\ \fIrepl\fB[\fR,\ \fIin\ \fB]\fR)" 10
.br
Substitute the string
.IR repl
in place of the first instance of the extended regular expression
.IR ERE
in string
.IR in
and return the number of substitutions. An
<ampersand>
(\c
.BR '&' )
appearing in the string
.IR repl
shall be replaced by the string from
.IR in
that matches the ERE. An
<ampersand>
preceded with a
<backslash>
shall be interpreted as the literal
<ampersand>
character. An occurrence of two consecutive
<backslash>
characters shall be interpreted as just a single literal
<backslash>
character. Any other occurrence of a
<backslash>
(for example, preceding any other character) shall be treated as a
literal
<backslash>
character. Note that if
.IR repl
is a string literal (the lexical token
.BR STRING ;
see
.IR "Grammar"),
the handling of the
<ampersand>
character occurs after any lexical processing, including any lexical
<backslash>-escape
sequence processing. If
.IR in
is specified and it is not an lvalue (see
.IR "Expressions in awk"),
the behavior is undefined. If
.IR in
is omitted,
.IR awk
shall use the current record ($0) in its place.
.IP "\fBsubstr\fR(\fIs\fR,\ \fIm\fB[\fR,\ \fIn\ \fB]\fR)" 10
.br
Return the at most
.IR n -character
substring of
.IR s
that begins at position
.IR m ,
numbering from 1. If
.IR n
is omitted, or if
.IR n
specifies more characters than are left in the string, the length of
the substring shall be limited by the length of the string
.IR s .
.IP "\fBtolower\fR(\fIs\fR)" 10
Return a string based on the string
.IR s .
Each character in
.IR s
that is an uppercase letter specified to have a
.BR tolower
mapping by the
.IR LC_CTYPE
category of the current locale shall be replaced in the returned string
by the lowercase letter specified by the mapping. Other characters in
.IR s
shall be unchanged in the returned string.
.IP "\fBtoupper\fR(\fIs\fR)" 10
Return a string based on the string
.IR s .
Each character in
.IR s
that is a lowercase letter specified to have a
.BR toupper
mapping by the
.IR LC_CTYPE
category of the current locale is replaced in the returned string by
the uppercase letter specified by the mapping. Other characters in
.IR s
are unchanged in the returned string.
.P
All of the preceding functions that take
.IR ERE
as a parameter expect a pattern or a string valued expression that is a
regular expression as defined in
.IR "Regular Expressions".
.SS "Input/Output and General Functions"
.P
The input/output and general functions are:
.IP "\fBclose\fR(\fIexpression\fR)" 10
.br
Close the file or pipe opened by a
.BR print
or
.BR printf
statement or a call to
.BR getline
with the same string-valued
.IR expression .
The limit on the number of open
.IR expression
arguments is implementation-defined. If the close was successful, the
function shall return zero; otherwise, it shall return non-zero.
.IP "\fIexpression\ |\ \fBgetline\ [\fIvar\fB]\fR" 10
.br
Read a record of input from a stream piped from the output of a
command. The stream shall be created if no stream is currently open
with the value of
.IR expression
as its command name. The stream created shall be equivalent to one
created by a call to the
\fIpopen\fR()
function with the value of
.IR expression
as the
.IR command
argument and a value of
.IR r
as the
.IR mode
argument. As long as the stream remains open, subsequent calls in which
.IR expression
evaluates to the same string value shall read subsequent records from
the stream. The stream shall remain open until the
.BR close
function is called with an expression that evaluates to the same string
value. At that time, the stream shall be closed as if by a call to the
\fIpclose\fR()
function. If
.IR var
is omitted, $0 and
.BR NF
shall be set; otherwise,
.IR var
shall be set and, if appropriate, it shall be considered a numeric
string (see
.IR "Expressions in awk").
.RS 10 
.P
The
.BR getline
operator can form ambiguous constructs when there are unparenthesized
operators (including concatenate) to the left of the
.BR '|' 
(to the beginning of the expression containing
.BR getline ).
In the context of the
.BR '$' 
operator,
.BR '|' 
shall behave as if it had a lower precedence than
.BR '$' .
The result of evaluating other operators is unspecified, and conforming
applications shall parenthesize properly all such usages.
.RE
.IP "\fBgetline\fR" 10
Set $0 to the next input record from the current input file. This form
of
.BR getline
shall set the
.BR NF ,
.BR NR ,
and
.BR FNR
variables.
.IP "\fBgetline\ \fIvar\fR" 10
Set variable
.IR var
to the next input record from the current input file and, if
appropriate,
.IR var
shall be considered a numeric string (see
.IR "Expressions in awk").
This form of
.BR getline
shall set the
.BR FNR
and
.BR NR
variables.
.IP "\fBgetline\ \fB[\fIvar\fB]\ \fR<\ \fIexpression\fR" 10
.br
Read the next record of input from a named file. The
.IR expression
shall be evaluated to produce a string that is used as a pathname.
If the file of that name is not currently open, it shall be opened. As
long as the stream remains open, subsequent calls in which
.IR expression
evaluates to the same string value shall read subsequent records from
the file. The file shall remain open until the
.BR close
function is called with an expression that evaluates to the same string
value. If
.IR var
is omitted, $0 and
.BR NF
shall be set; otherwise,
.IR var
shall be set and, if appropriate, it shall be considered a numeric
string (see
.IR "Expressions in awk").
.RS 10 
.P
The
.BR getline
operator can form ambiguous constructs when there are unparenthesized
binary operators (including concatenate) to the right of the
.BR '<' 
(up to the end of the expression containing the
.BR getline ).
The result of evaluating such a construct is unspecified, and conforming
applications shall parenthesize properly all such usages.
.RE
.IP "\fBsystem\fR(\fIexpression\fR)" 10
.br
Execute the command given by
.IR expression
in a manner equivalent to the
\fIsystem\fR()
function defined in the System Interfaces volume of POSIX.1\(hy2017 and return the exit status of the
command.
.P
All forms of
.BR getline
shall return 1 for successful input, zero for end-of-file, and \-1
for an error.
.P
Where strings are used as the name of a file or pipeline, the
application shall ensure that the strings are textually identical. The
terminology ``same string value'' implies that ``equivalent strings'',
even those that differ only by
<space>
characters, represent different files.
.SS "User-Defined Functions"
.P
The
.IR awk
language also provides user-defined functions. Such functions can be
defined as:
.sp
.RS 4
.nf

function \fIname\fR(\fB[\fIparameter\fR, ...\fB]\fR) { \fIstatements\fR }
.fi
.P
.RE
.P
A function can be referred to anywhere in an
.IR awk
program; in particular, its use can precede its definition. The scope
of a function is global.
.P
Function parameters, if present, can be either scalars or arrays; the
behavior is undefined if an array name is passed as a parameter that
the function uses as a scalar, or if a scalar expression is passed as a
parameter that the function uses as an array. Function parameters shall
be passed by value if scalar and by reference if array name.
.P
The number of parameters in the function definition need not match the
number of parameters in the function call. Excess formal parameters can
be used as local variables. If fewer arguments are supplied in a
function call than are in the function definition, the extra parameters
that are used in the function body as scalars shall evaluate to the
uninitialized value until they are otherwise initialized, and the extra
parameters that are used in the function body as arrays shall be
treated as uninitialized arrays where each element evaluates to the
uninitialized value until otherwise initialized.
.P
When invoking a function, no white space can be placed between the
function name and the opening parenthesis. Function calls can be nested
and recursive calls can be made upon functions. Upon return from any
nested or recursive function call, the values of all of the calling
function's parameters shall be unchanged, except for array parameters
passed by reference. The
.BR return
statement can be used to return a value. If a
.BR return
statement appears outside of a function definition, the behavior is
undefined.
.P
In the function definition,
<newline>
characters shall be optional before the opening brace and after the
closing brace. Function definitions can appear anywhere in the program
where a
.IR pattern-action
pair is allowed.
.SS "Grammar"
.P
The grammar in this section and the lexical conventions in the
following section shall together describe the syntax for
.IR awk
programs. The general conventions for this style of grammar are
described in
.IR "Section 1.3" ", " "Grammar Conventions".
A valid program can be represented as the non-terminal symbol
.IR program
in the grammar. This formal syntax shall take precedence over the
preceding text syntax description.
.sp
.RS 4
.nf

%token NAME NUMBER STRING ERE
%token FUNC_NAME   /* Name followed by \(aq(\(aq without white space. */
.P
/* Keywords */
%token       Begin   End
/*          \(aqBEGIN\(aq \(aqEND\(aq                            */
.P
%token       Break   Continue   Delete   Do   Else
/*          \(aqbreak\(aq \(aqcontinue\(aq \(aqdelete\(aq \(aqdo\(aq \(aqelse\(aq  */
.P
%token       Exit   For   Function   If   In
/*          \(aqexit\(aq \(aqfor\(aq \(aqfunction\(aq \(aqif\(aq \(aqin\(aq        */
.P
%token       Next   Print   Printf   Return   While
/*          \(aqnext\(aq \(aqprint\(aq \(aqprintf\(aq \(aqreturn\(aq \(aqwhile\(aq */
.P
/* Reserved function names */
%token BUILTIN_FUNC_NAME
            /* One token for the following:
             * atan2 cos sin exp log sqrt int rand srand
             * gsub index length match split sprintf sub
             * substr tolower toupper close system
             */
%token GETLINE
            /* Syntactically different from other built-ins. */
.P
/* Two-character tokens. */
%token ADD_ASSIGN SUB_ASSIGN MUL_ASSIGN DIV_ASSIGN MOD_ASSIGN POW_ASSIGN
/*     \(aq+=\(aq       \(aq-=\(aq       \(aq*=\(aq       \(aq/=\(aq       \(aq%=\(aq       \(aq\(ha=\(aq */
.P
%token OR   AND  NO_MATCH   EQ   LE   GE   NE   INCR  DECR  APPEND
/*     \(aq||\(aq \(aq&&\(aq \(aq!\^\(ti\(aq \(aq==\(aq \(aq<=\(aq \(aq>=\(aq \(aq!=\(aq \(aq++\(aq  \(aq--\(aq  \(aq>>\(aq   */
.P
/* One-character tokens. */
%token \(aq{\(aq \(aq}\(aq \(aq(\(aq \(aq)\(aq \(aq[\(aq \(aq]\(aq \(aq,\(aq \(aq;\(aq NEWLINE
%token \(aq+\(aq \(aq-\(aq \(aq*\(aq \(aq%\(aq \(aq\(ha\(aq \(aq!\(aq \(aq>\(aq \(aq<\(aq \(aq|\(aq \(aq?\(aq \(aq:\(aq \(aq\(ti\(aq \(aq$\(aq \(aq=\(aq
.P
%start program
%%
.P
program          : item_list
                 | item_list item
                 ;
.P
item_list        : /* empty */
                 | item_list item terminator
                 ;
.P
item             : action
                 | pattern action
                 | normal_pattern
                 | Function NAME      \(aq(\(aq param_list_opt \(aq)\(aq
                       newline_opt action
                 | Function FUNC_NAME \(aq(\(aq param_list_opt \(aq)\(aq
                       newline_opt action
                 ;
.P
param_list_opt   : /* empty */
                 | param_list
                 ;
.P
param_list       : NAME
                 | param_list \(aq,\(aq NAME
                 ;
.P
pattern          : normal_pattern
                 | special_pattern
                 ;
.P
normal_pattern   : expr
                 | expr \(aq,\(aq newline_opt expr
                 ;
.P
special_pattern  : Begin
                 | End
                 ;
.P
action           : \(aq{\(aq newline_opt                             \(aq}\(aq
                 | \(aq{\(aq newline_opt terminated_statement_list   \(aq}\(aq
                 | \(aq{\(aq newline_opt unterminated_statement_list \(aq}\(aq
                 ;
.P
terminator       : terminator NEWLINE
                 |            \(aq;\(aq
                 |            NEWLINE
                 ;
.P
terminated_statement_list : terminated_statement
                 | terminated_statement_list terminated_statement
                 ;
.P
unterminated_statement_list : unterminated_statement
                 | terminated_statement_list unterminated_statement
                 ;
.P
terminated_statement : action newline_opt
                 | If \(aq(\(aq expr \(aq)\(aq newline_opt terminated_statement
                 | If \(aq(\(aq expr \(aq)\(aq newline_opt terminated_statement
                       Else newline_opt terminated_statement
                 | While \(aq(\(aq expr \(aq)\(aq newline_opt terminated_statement
                 | For \(aq(\(aq simple_statement_opt \(aq;\(aq
                      expr_opt \(aq;\(aq simple_statement_opt \(aq)\(aq newline_opt
                      terminated_statement
                 | For \(aq(\(aq NAME In NAME \(aq)\(aq newline_opt
                      terminated_statement
                 | \(aq;\(aq newline_opt
                 | terminatable_statement NEWLINE newline_opt
                 | terminatable_statement \(aq;\(aq     newline_opt
                 ;
.P
unterminated_statement : terminatable_statement
                 | If \(aq(\(aq expr \(aq)\(aq newline_opt unterminated_statement
                 | If \(aq(\(aq expr \(aq)\(aq newline_opt terminated_statement
                      Else newline_opt unterminated_statement
                 | While \(aq(\(aq expr \(aq)\(aq newline_opt unterminated_statement
                 | For \(aq(\(aq simple_statement_opt \(aq;\(aq
                  expr_opt \(aq;\(aq simple_statement_opt \(aq)\(aq newline_opt
                      unterminated_statement
                 | For \(aq(\(aq NAME In NAME \(aq)\(aq newline_opt
                      unterminated_statement
                 ;
.P
terminatable_statement : simple_statement
                 | Break
                 | Continue
                 | Next
                 | Exit expr_opt
                 | Return expr_opt
                 | Do newline_opt terminated_statement While \(aq(\(aq expr \(aq)\(aq
                 ;
.P
simple_statement_opt : /* empty */
                 | simple_statement
                 ;
.P
simple_statement : Delete NAME \(aq[\(aq expr_list \(aq]\(aq
                 | expr
                 | print_statement
                 ;
.P
print_statement  : simple_print_statement
                 | simple_print_statement output_redirection
                 ;
.P
simple_print_statement : Print  print_expr_list_opt
                 | Print  \(aq(\(aq multiple_expr_list \(aq)\(aq
                 | Printf print_expr_list
                 | Printf \(aq(\(aq multiple_expr_list \(aq)\(aq
                 ;
.P
output_redirection : \(aq>\(aq    expr
                 | APPEND expr
                 | \(aq|\(aq    expr
                 ;
.P
expr_list_opt    : /* empty */
                 | expr_list
                 ;
.P
expr_list        : expr
                 | multiple_expr_list
                 ;
.P
multiple_expr_list : expr \(aq,\(aq newline_opt expr
                 | multiple_expr_list \(aq,\(aq newline_opt expr
                 ;
.P
expr_opt         : /* empty */
                 | expr
                 ;
.P
expr             : unary_expr
                 | non_unary_expr
                 ;
.P
unary_expr       : \(aq+\(aq expr
                 | \(aq-\(aq expr
                 | unary_expr \(aq\(ha\(aq      expr
                 | unary_expr \(aq*\(aq      expr
                 | unary_expr \(aq/\(aq      expr
                 | unary_expr \(aq%\(aq      expr
                 | unary_expr \(aq+\(aq      expr
                 | unary_expr \(aq-\(aq      expr
                 | unary_expr          non_unary_expr
                 | unary_expr \(aq<\(aq      expr
                 | unary_expr LE       expr
                 | unary_expr NE       expr
                 | unary_expr EQ       expr
                 | unary_expr \(aq>\(aq      expr
                 | unary_expr GE       expr
                 | unary_expr \(aq\(ti\(aq      expr
                 | unary_expr NO_MATCH expr
                 | unary_expr In NAME
                 | unary_expr AND newline_opt expr
                 | unary_expr OR  newline_opt expr
                 | unary_expr \(aq?\(aq expr \(aq:\(aq expr
                 | unary_input_function
                 ;
.P
non_unary_expr   : \(aq(\(aq expr \(aq)\(aq
                 | \(aq!\(aq expr
                 | non_unary_expr \(aq\(ha\(aq      expr
                 | non_unary_expr \(aq*\(aq      expr
                 | non_unary_expr \(aq/\(aq      expr
                 | non_unary_expr \(aq%\(aq      expr
                 | non_unary_expr \(aq+\(aq      expr
                 | non_unary_expr \(aq-\(aq      expr
                 | non_unary_expr          non_unary_expr
                 | non_unary_expr \(aq<\(aq      expr
                 | non_unary_expr LE       expr
                 | non_unary_expr NE       expr
                 | non_unary_expr EQ       expr
                 | non_unary_expr \(aq>\(aq      expr
                 | non_unary_expr GE       expr
                 | non_unary_expr \(aq\(ti\(aq      expr
                 | non_unary_expr NO_MATCH expr
                 | non_unary_expr In NAME
                 | \(aq(\(aq multiple_expr_list \(aq)\(aq In NAME
                 | non_unary_expr AND newline_opt expr
                 | non_unary_expr OR  newline_opt expr
                 | non_unary_expr \(aq?\(aq expr \(aq:\(aq expr
                 | NUMBER
                 | STRING
                 | lvalue
                 | ERE
                 | lvalue INCR
                 | lvalue DECR
                 | INCR lvalue
                 | DECR lvalue
                 | lvalue POW_ASSIGN expr
                 | lvalue MOD_ASSIGN expr
                 | lvalue MUL_ASSIGN expr
                 | lvalue DIV_ASSIGN expr
                 | lvalue ADD_ASSIGN expr
                 | lvalue SUB_ASSIGN expr
                 | lvalue \(aq=\(aq expr
                 | FUNC_NAME \(aq(\(aq expr_list_opt \(aq)\(aq
                      /* no white space allowed before \(aq(\(aq */
                 | BUILTIN_FUNC_NAME \(aq(\(aq expr_list_opt \(aq)\(aq
                 | BUILTIN_FUNC_NAME
                 | non_unary_input_function
                 ;
.P
print_expr_list_opt : /* empty */
                 | print_expr_list
                 ;
.P
print_expr_list  : print_expr
                 | print_expr_list \(aq,\(aq newline_opt print_expr
                 ;
.P
print_expr       : unary_print_expr
                 | non_unary_print_expr
                 ;
.P
unary_print_expr : \(aq+\(aq print_expr
                 | \(aq-\(aq print_expr
                 | unary_print_expr \(aq\(ha\(aq      print_expr
                 | unary_print_expr \(aq*\(aq      print_expr
                 | unary_print_expr \(aq/\(aq      print_expr
                 | unary_print_expr \(aq%\(aq      print_expr
                 | unary_print_expr \(aq+\(aq      print_expr
                 | unary_print_expr \(aq-\(aq      print_expr
                 | unary_print_expr          non_unary_print_expr
                 | unary_print_expr \(aq\(ti\(aq      print_expr
                 | unary_print_expr NO_MATCH print_expr
                 | unary_print_expr In NAME
                 | unary_print_expr AND newline_opt print_expr
                 | unary_print_expr OR  newline_opt print_expr
                 | unary_print_expr \(aq?\(aq print_expr \(aq:\(aq print_expr
                 ;
.P
non_unary_print_expr : \(aq(\(aq expr \(aq)\(aq
                 | \(aq!\(aq print_expr
                 | non_unary_print_expr \(aq\(ha\(aq      print_expr
                 | non_unary_print_expr \(aq*\(aq      print_expr
                 | non_unary_print_expr \(aq/\(aq      print_expr
                 | non_unary_print_expr \(aq%\(aq      print_expr
                 | non_unary_print_expr \(aq+\(aq      print_expr
                 | non_unary_print_expr \(aq-\(aq      print_expr
                 | non_unary_print_expr          non_unary_print_expr
                 | non_unary_print_expr \(aq\(ti\(aq      print_expr
                 | non_unary_print_expr NO_MATCH print_expr
                 | non_unary_print_expr In NAME
                 | \(aq(\(aq multiple_expr_list \(aq)\(aq In NAME
                 | non_unary_print_expr AND newline_opt print_expr
                 | non_unary_print_expr OR  newline_opt print_expr
                 | non_unary_print_expr \(aq?\(aq print_expr \(aq:\(aq print_expr
                 | NUMBER
                 | STRING
                 | lvalue
                 | ERE
                 | lvalue INCR
                 | lvalue DECR
                 | INCR lvalue
                 | DECR lvalue
                 | lvalue POW_ASSIGN print_expr
                 | lvalue MOD_ASSIGN print_expr
                 | lvalue MUL_ASSIGN print_expr
                 | lvalue DIV_ASSIGN print_expr
                 | lvalue ADD_ASSIGN print_expr
                 | lvalue SUB_ASSIGN print_expr
                 | lvalue \(aq=\(aq print_expr
                 | FUNC_NAME \(aq(\(aq expr_list_opt \(aq)\(aq
                     /* no white space allowed before \(aq(\(aq */
                 | BUILTIN_FUNC_NAME \(aq(\(aq expr_list_opt \(aq)\(aq
                 | BUILTIN_FUNC_NAME
                 ;
.P
lvalue           : NAME
                 | NAME \(aq[\(aq expr_list \(aq]\(aq
                 | \(aq$\(aq expr
                 ;
.P
non_unary_input_function : simple_get
                 | simple_get \(aq<\(aq expr
                 | non_unary_expr \(aq|\(aq simple_get
                 ;
.P
unary_input_function : unary_expr \(aq|\(aq simple_get
                 ;
.P
simple_get       : GETLINE
                 | GETLINE lvalue
                 ;
.P
newline_opt      : /* empty */
                 | newline_opt NEWLINE
                 ;
.fi
.P
.RE
.P
This grammar has several ambiguities that shall be resolved as
follows:
.IP " *" 4
Operator precedence and associativity shall be as described in
.IR "Table 4-1, Expressions in Decreasing Precedence in \fIawk\fP".
.IP " *" 4
In case of ambiguity, an
.BR else
shall be associated with the most immediately preceding
.BR if
that would satisfy the grammar.
.IP " *" 4
In some contexts, a
<slash>
(\c
.BR '/' )
that is used to surround an ERE could also be the division operator.
This shall be resolved in such a way that wherever the division
operator could appear, a
<slash>
is assumed to be the division operator. (There is no unary division
operator.)
.P
Each expression in an
.IR awk
program shall conform to the precedence and associativity rules, even
when this is not needed to resolve an ambiguity. For example, because
.BR '$' 
has higher precedence than
.BR '++' ,
the string
.BR \(dq$x++--\(dq 
is not a valid
.IR awk
expression, even though it is unambiguously parsed by the grammar as
.BR \(dq$(x++)--\(dq .
.P
One convention that might not be obvious from the formal grammar is
where
<newline>
characters are acceptable. There are several obvious placements such as
terminating a statement, and a
<backslash>
can be used to escape
<newline>
characters between any lexical tokens. In addition,
<newline>
characters without
<backslash>
characters can follow a comma, an open brace, logical AND operator (\c
.BR \(dq&&\(dq ),
logical OR operator (\c
.BR \(dq||\(dq ),
the
.BR do
keyword, the
.BR else
keyword, and the closing parenthesis of an
.BR if ,
.BR for ,
or
.BR while
statement. For example:
.sp
.RS 4
.nf

{ print  $1,
         $2 }
.fi
.P
.RE
.SS "Lexical Conventions"
.P
The lexical conventions for
.IR awk
programs, with respect to the preceding grammar, shall be as follows:
.IP " 1." 4
Except as noted,
.IR awk
shall recognize the longest possible token or delimiter beginning at a
given point.
.IP " 2." 4
A comment shall consist of any characters beginning with the
<number-sign>
character and terminated by, but excluding the next occurrence of, a
<newline>.
Comments shall have no effect, except to delimit lexical tokens.
.IP " 3." 4
The
<newline>
shall be recognized as the token
.BR NEWLINE .
.IP " 4." 4
A
<backslash>
character immediately followed by a
<newline>
shall have no effect.
.IP " 5." 4
The token
.BR STRING
shall represent a string constant. A string constant shall begin with
the character
.BR '\&"' .
Within a string constant, a
<backslash>
character shall be considered to begin an escape sequence as specified
in the table in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 5" ", " "File Format Notation"
(\c
.BR '\e\e' ,
.BR '\ea' ,
.BR '\eb' ,
.BR '\ef' ,
.BR '\en' ,
.BR '\er' ,
.BR '\et' ,
.BR '\ev' ).
In addition, the escape sequences in
.IR "Table 4-2, Escape Sequences in \fIawk\fP"
shall be recognized. A
<newline>
shall not occur within a string constant. A string constant shall be
terminated by the first unescaped occurrence of the character
.BR '\&"' 
after the one that begins the string constant. The value of the string
shall be the sequence of all unescaped characters and values of escape
sequences between, but not including, the two delimiting
.BR '\&"' 
characters.
.IP " 6." 4
The token
.BR ERE
represents an extended regular expression constant. An ERE constant
shall begin with the
<slash>
character. Within an ERE constant, a
<backslash>
character shall be considered to begin an escape sequence as
specified in the table in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 5" ", " "File Format Notation".
In addition, the escape sequences in
.IR "Table 4-2, Escape Sequences in \fIawk\fP"
shall be recognized. The application shall ensure that a
<newline>
does not occur within an ERE constant. An ERE constant shall be
terminated by the first unescaped occurrence of the
<slash>
character after the one that begins the ERE constant. The extended regular
expression represented by the ERE constant shall be the sequence of all
unescaped characters and values of escape sequences between, but not
including, the two delimiting
<slash>
characters.
.IP " 7." 4
A
<blank>
shall have no effect, except to delimit lexical tokens or within
.BR STRING
or
.BR ERE
tokens.
.IP " 8." 4
The token
.BR NUMBER
shall represent a numeric constant. Its form and numeric value shall
either be equivalent to the
.BR decimal-floating-constant
token as specified by the ISO\ C standard, or it shall be a sequence of decimal
digits and shall be evaluated as an integer constant in decimal. In
addition, implementations may accept numeric constants with the form
and numeric value equivalent to the
.BR hexadecimal-constant
and
.BR hexadecimal-floating-constant
tokens as specified by the ISO\ C standard.
.RS 4 
.P
If the value is too large or too small to be representable (see
.IR "Section 1.1.2" ", " "Concepts Derived from the ISO C Standard"),
the behavior is undefined.
.RE
.IP " 9." 4
A sequence of underscores, digits, and alphabetics from the portable
character set (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 6.1" ", " "Portable Character Set"),
beginning with an
<underscore>
or alphabetic character, shall be considered a word.
.IP 10. 4
The following words are keywords that shall be recognized as individual
tokens; the name of the token is the same as the keyword:
.TS
tab(@);
lw(0.6i)eB leB leB leB leB leB.
T{
.nf
BEGIN
break
continue
T}@T{
.nf
delete
do
else
T}@T{
.nf
END
exit
for
T}@T{
.nf
function
getline
if
T}@T{
.nf
in
next
print
T}@T{
.nf
printf
return
while
T}
.TE
.IP 11. 4
The following words are names of built-in functions and shall be
recognized as the token
.BR BUILTIN_FUNC_NAME :
.TS
tab(@);
lw(0.6i)eB leB leB leB leB leB.
T{
.nf
atan2
close
cos
exp
T}@T{
.nf
gsub
index
int
length
T}@T{
.nf
log
match
rand
sin
T}@T{
.nf
split
sprintf
sqrt
srand
T}@T{
.nf
sub
substr
system
tolower
T}@T{
.nf
toupper
.fi
T}
.TE
.RS 4 
.P
The above-listed keywords and names of built-in functions are
considered reserved words.
.RE
.IP 12. 4
The token
.BR NAME
shall consist of a word that is not a keyword or a name of a built-in
function and is not followed immediately (without any delimiters) by
the
.BR '(' 
character.
.IP 13. 4
The token
.BR FUNC_NAME
shall consist of a word that is not a keyword or a name of a built-in
function, followed immediately (without any delimiters) by the
.BR '(' 
character. The
.BR '(' 
character shall not be included as part of the token.
.IP 14. 4
The following two-character sequences shall be recognized as the named
tokens:
.TS
box center tab(@);
cB | cB | cB | cB
lB | cf5 | lB | cf5.
Token Name@Sequence@Token Name@Sequence
_
ADD_ASSIGN@+=@NO_MATCH@!~
SUB_ASSIGN@\-=@EQ@==
MUL_ASSIGN@*=@LE@<=
DIV_ASSIGN@/=@GE@>=
MOD_ASSIGN@%=@NE@!=
POW_ASSIGN@^=@INCR@++
OR@||@DECR@\-\|\-
AND@&&@APPEND@>>
.TE
.IP 15. 4
The following single characters shall be recognized as tokens whose
names are the character:
.RS 4 
.sp
.RS 4
.nf

<newline> { } ( ) [ ] , ; + - * % \(ha ! > < | ? : \(ti $ =
.fi
.P
.RE
.RE
.P
There is a lexical ambiguity between the token
.BR ERE
and the tokens
.BR '/' 
and
.BR DIV_ASSIGN .
When an input sequence begins with a
<slash>
character in any syntactic context where the token
.BR '/' 
or
.BR DIV_ASSIGN
could appear as the next token in a valid program, the longer of those
two tokens that can be recognized shall be recognized. In any other
syntactic context where the token
.BR ERE
could appear as the next token in a valid program, the token
.BR ERE
shall be recognized.
.SH "EXIT STATUS"
The following exit values shall be returned:
.IP "\00" 6
All input files were processed successfully.
.IP >0 6
An error occurred.
.P
The exit status can be altered within the program by using an
.BR exit
expression.
.SH "CONSEQUENCES OF ERRORS"
If any
.IR file
operand is specified and the named file cannot be accessed,
.IR awk
shall write a diagnostic message to standard error and terminate
without any further action.
.P
If the program specified by either the
.IR program
operand or a
.IR progfile
operand is not a valid
.IR awk
program (as specified in the EXTENDED DESCRIPTION section), the
behavior is undefined.
.LP
.IR "The following sections are informative."
.SH "APPLICATION USAGE"
The
.BR index ,
.BR length ,
.BR match ,
and
.BR substr
functions should not be confused with similar functions in the ISO\ C standard;
the
.IR awk
versions deal with characters, while the ISO\ C standard deals with bytes.
.P
Because the concatenation operation is represented by adjacent
expressions rather than an explicit operator, it is often necessary to
use parentheses to enforce the proper evaluation precedence.
.P
When using
.IR awk
to process pathnames, it is recommended that LC_ALL, or at least
LC_CTYPE and LC_COLLATE, are set to POSIX or C in the environment,
since pathnames can contain byte sequences that do not form valid
characters in some locales, in which case the utility's behavior would
be undefined. In the POSIX locale each byte is a valid single-byte
character, and therefore this problem is avoided.
.P
On implementations where the
.BR \(dq==\(dq 
operator checks if strings collate equally, applications needing to
check whether strings are identical can use:
.sp
.RS 4
.nf

length(a) == length(b) && index(a,b) == 1
.fi
.P
.RE
.P
On implementations where the
.BR \(dq==\(dq 
operator checks if strings are identical, applications needing to
check whether strings collate equally can use:
.sp
.RS 4
.nf

a <= b && a >= b
.fi
.P
.RE
.SH EXAMPLES
The
.IR awk
program specified in the command line is most easily specified within
single-quotes (for example, \(aq\fIprogram\fP\(aq) for applications using
.IR sh ,
because
.IR awk
programs commonly contain characters that are special to the shell,
including double-quotes. In the cases where an
.IR awk
program contains single-quote characters, it is usually easiest to
specify most of the program as strings within single-quotes
concatenated by the shell with quoted single-quote characters. For
example:
.sp
.RS 4
.nf

awk \(aq/\(aq\e\(aq\(aq/ { print "quote:", $0 }\(aq
.fi
.P
.RE
.P
prints all lines from the standard input containing a single-quote
character, prefixed with
.IR quote :.
.P
The following are examples of simple
.IR awk
programs:
.IP " 1." 4
Write to the standard output all input lines for which field 3 is
greater than 5:
.RS 4 
.sp
.RS 4
.nf

$3 > 5
.fi
.P
.RE
.RE
.IP " 2." 4
Write every tenth line:
.RS 4 
.sp
.RS 4
.nf

(NR % 10) == 0
.fi
.P
.RE
.RE
.IP " 3." 4
Write any line with a substring matching the regular expression:
.RS 4 
.sp
.RS 4
.nf

/(G|D)(2[0-9][[:alpha:]]*)/
.fi
.P
.RE
.RE
.IP " 4." 4
Print any line with a substring containing a
.BR 'G' 
or
.BR 'D' ,
followed by a sequence of digits and characters. This example uses
character classes
.BR digit
and
.BR alpha
to match language-independent digit and alphabetic characters
respectively:
.RS 4 
.sp
.RS 4
.nf

/(G|D)([[:digit:][:alpha:]]*)/
.fi
.P
.RE
.RE
.IP " 5." 4
Write any line in which the second field matches the regular expression
and the fourth field does not:
.RS 4 
.sp
.RS 4
.nf

$2 \(ti /xyz/ && $4 !\(ti /xyz/
.fi
.P
.RE
.RE
.IP " 6." 4
Write any line in which the second field contains a
<backslash>:
.RS 4 
.sp
.RS 4
.nf

$2 \(ti /\e\e/
.fi
.P
.RE
.RE
.IP " 7." 4
Write any line in which the second field contains a
<backslash>.
Note that
<backslash>-escapes
are interpreted twice; once in lexical processing of the string and once
in processing the regular expression:
.RS 4 
.sp
.RS 4
.nf

$2 \(ti "\e\e\e\e"
.fi
.P
.RE
.RE
.IP " 8." 4
Write the second to the last and the last field in each line. Separate
the fields by a
<colon>:
.RS 4 
.sp
.RS 4
.nf

{OFS=":";print $(NF-1), $NF}
.fi
.P
.RE
.RE
.IP " 9." 4
Write the line number and number of fields in each line. The three
strings representing the line number, the
<colon>,
and the number of fields are concatenated and that string is written to
standard output:
.RS 4 
.sp
.RS 4
.nf

{print NR ":" NF}
.fi
.P
.RE
.RE
.IP 10. 4
Write lines longer than 72 characters:
.RS 4 
.sp
.RS 4
.nf

length($0) > 72
.fi
.P
.RE
.RE
.IP 11. 4
Write the first two fields in opposite order separated by
.BR OFS :
.RS 4 
.sp
.RS 4
.nf

{ print $2, $1 }
.fi
.P
.RE
.RE
.IP 12. 4
Same, with input fields separated by a
<comma>
or
<space>
and
<tab>
characters, or both:
.RS 4 
.sp
.RS 4
.nf

BEGIN { FS = ",[ \et]*|[ \et]+" }
      { print $2, $1 }
.fi
.P
.RE
.RE
.IP 13. 4
Add up the first column, print sum, and average:
.RS 4 
.sp
.RS 4
.nf

      {s += $1 }
END   {print "sum is ", s, " average is", s/NR}
.fi
.P
.RE
.RE
.IP 14. 4
Write fields in reverse order, one per line (many lines out for each
line in):
.RS 4 
.sp
.RS 4
.nf

{ for (i = NF; i > 0; --i) print $i }
.fi
.P
.RE
.RE
.IP 15. 4
Write all lines between occurrences of the strings
.BR start
and
.BR stop :
.RS 4 
.sp
.RS 4
.nf

/start/, /stop/
.fi
.P
.RE
.RE
.IP 16. 4
Write all lines whose first field is different from the previous one:
.RS 4 
.sp
.RS 4
.nf

$1 != prev { print; prev = $1 }
.fi
.P
.RE
.RE
.IP 17. 4
Simulate
.IR echo :
.RS 4 
.sp
.RS 4
.nf

BEGIN  {
        for (i = 1; i < ARGC; ++i)
        printf("%s%s", ARGV[i], i==ARGC-1?"\en":" ")
}
.fi
.P
.RE
.RE
.IP 18. 4
Write the path prefixes contained in the
.IR PATH
environment variable, one per line:
.RS 4 
.sp
.RS 4
.nf

BEGIN  {
        n = split (ENVIRON["PATH"], path, ":")
        for (i = 1; i <= n; ++i)
        print path[i]
}
.fi
.P
.RE
.RE
.IP 19. 4
If there is a file named
.BR input
containing page headers of the form:
Page #
.RS 4 
.P
and a file named
.BR program
that contains:
.sp
.RS 4
.nf

/Page/   { $2 = n++; }
         { print }
.fi
.P
.RE
then the command line:
.sp
.RS 4
.nf

awk -f program n=5 input
.fi
.P
.RE
.P
prints the file
.BR input ,
filling in page numbers starting at 5.
.RE
.SH RATIONALE
This description is based on the new
.IR awk ,
``nawk'', (see the referenced \fIThe AWK Programming Language\fP), which introduced a number of new features to
the historical
.IR awk :
.IP " 1." 4
New keywords:
.BR delete ,
.BR do ,
.BR function ,
.BR return
.IP " 2." 4
New built-in functions:
.BR atan2 ,
.BR close ,
.BR cos ,
.BR gsub ,
.BR match ,
.BR rand ,
.BR sin ,
.BR srand ,
.BR sub ,
.BR system
.IP " 3." 4
New predefined variables:
.BR FNR ,
.BR ARGC ,
.BR ARGV ,
.BR RSTART ,
.BR RLENGTH ,
.BR SUBSEP
.IP " 4." 4
New expression operators:
.BR ? ,
.BR : ,
.BR , ,
.BR ^
.IP " 5." 4
The
.BR FS
variable and the third argument to
.BR split ,
now treated as extended regular expressions.
.IP " 6." 4
The operator precedence, changed to more closely match the C language.
Two examples of code that operate differently are:
.RS 4 
.sp
.RS 4
.nf

while ( n /= 10 > 1) ...
if (!"wk" \(ti /bwk/) ...
.fi
.P
.RE
.RE
.P
Several features have been added based on newer implementations of
.IR awk :
.IP " *" 4
Multiple instances of
.BR \-f
.IR progfile
are permitted.
.IP " *" 4
The new option
.BR \-v
.IR assignment.
.IP " *" 4
The new predefined variable
.BR ENVIRON .
.IP " *" 4
New built-in functions
.BR toupper
and
.BR tolower .
.IP " *" 4
More formatting capabilities are added to
.BR printf
to match the ISO\ C standard.
.P
Earlier versions of this standard required implementations to
support multiple adjacent
<semicolon>s,
lines with one or more
<semicolon>
before a rule (\c
.IR pattern-action
pairs), and lines with only
<semicolon>(s).
These are not required by this standard and are considered poor
programming practice, but can be accepted by an implementation of
.IR awk
as an extension.
.P
The overall
.IR awk
syntax has always been based on the C language, with a few features
from the shell command language and other sources. Because of this, it
is not completely compatible with any other language, which has caused
confusion for some users. It is not the intent of the standard
developers to address such issues. A few relatively minor changes
toward making the language more compatible with the ISO\ C standard were
made; most of these changes are based on similar changes in recent
implementations, as described above. There remain several C-language
conventions that are not in
.IR awk .
One of the notable ones is the
<comma>
operator, which is commonly used to specify multiple expressions in the
C language
.BR for
statement. Also, there are various places where
.IR awk
is more restrictive than the C language regarding the type of
expression that can be used in a given context. These limitations are
due to the different features that the
.IR awk
language does provide.
.P
Regular expressions in
.IR awk
have been extended somewhat from historical implementations to make
them a pure superset of extended regular expressions, as defined by
POSIX.1\(hy2008 (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 9.4" ", " "Extended Regular Expressions").
The main extensions are internationalization
features and interval expressions. Historical implementations of
.IR awk
have long supported
<backslash>-escape
sequences as an extension to extended regular expressions, and
this extension has been retained despite inconsistency with other
utilities. The number of escape sequences recognized in both extended
regular expressions and strings has varied (generally increasing with
time) among implementations. The set specified by POSIX.1\(hy2008 includes most
sequences known to be supported by popular implementations and by the
ISO\ C standard. One sequence that is not supported is hexadecimal value escapes
beginning with
.BR '\ex' .
This would allow values expressed in more than 9 bits to be used within
.IR awk
as in the ISO\ C standard. However, because this syntax has a non-deterministic
length, it does not permit the subsequent character to be a hexadecimal
digit. This limitation can be dealt with in the C language by the use
of lexical string concatenation. In the
.IR awk
language, concatenation could also be a solution for strings, but not
for extended regular expressions (either lexical ERE tokens or strings
used dynamically as regular expressions). Because of this limitation,
the feature has not been added to POSIX.1\(hy2008.
.P
When a string variable is used in a context where an extended regular
expression normally appears (where the lexical token ERE is used in the
grammar) the string does not contain the literal
<slash>
characters.
.P
Some versions of
.IR awk
allow the form:
.sp
.RS 4
.nf

func name(args, ... ) { statements }
.fi
.P
.RE
.P
This has been deprecated by the authors of the language, who asked that
it not be specified.
.P
Historical implementations of
.IR awk
produce an error if a
.BR next
statement is executed in a
.BR BEGIN
action, and cause
.IR awk
to terminate if a
.BR next
statement is executed in an
.BR END
action. This behavior has not been documented, and it was not believed
that it was necessary to standardize it.
.P
The specification of conversions between string and numeric values is
much more detailed than in the documentation of historical
implementations or in the referenced \fIThe AWK Programming Language\fP. Although most of the behavior is
designed to be intuitive, the details are necessary to ensure
compatible behavior from different implementations. This is especially
important in relational expressions since the types of the operands
determine whether a string or numeric comparison is performed. From the
perspective of an application developer, it is usually sufficient to
expect intuitive behavior and to force conversions (by adding zero or
concatenating a null string) when the type of an expression does not
obviously match what is needed. The intent has been to specify
historical practice in almost all cases. The one exception is that, in
historical implementations, variables and constants maintain both
string and numeric values after their original value is converted by
any use. This means that referencing a variable or constant can have
unexpected side-effects. For example, with historical implementations
the following program:
.sp
.RS 4
.nf

{
    a = "+2"
    b = 2
    if (NR % 2)
        c = a + b
    if (a == b)
        print "numeric comparison"
    else
        print "string comparison"
}
.fi
.P
.RE
.P
would perform a numeric comparison (and output numeric comparison) for
each odd-numbered line, but perform a string comparison (and output
string comparison) for each even-numbered line. POSIX.1\(hy2008 ensures that
comparisons will be numeric if necessary. With historical
implementations, the following program:
.sp
.RS 4
.nf

BEGIN {
    OFMT = "%e"
    print 3.14
    OFMT = "%f"
    print 3.14
}
.fi
.P
.RE
.P
would output
.BR \(dq3.140000e+00\(dq 
twice, because in the second
.BR print
statement the constant
.BR \(dq3.14\(dq 
would have a string value from the previous conversion. POSIX.1\(hy2008 requires
that the output of the second
.BR print
statement be
.BR \(dq3.140000\(dq .
The behavior of historical implementations was seen as too unintuitive
and unpredictable.
.P
It was pointed out that with the rules contained in early drafts, the
following script would print nothing:
.sp
.RS 4
.nf

BEGIN {
    y[1.5] = 1
    OFMT = "%e"
    print y[1.5]
}
.fi
.P
.RE
.P
Therefore, a new variable,
.BR CONVFMT ,
was introduced. The
.BR OFMT
variable is now restricted to affecting output conversions of numbers
to strings and
.BR CONVFMT
is used for internal conversions, such as comparisons or array
indexing. The default value is the same as that for
.BR OFMT ,
so unless a program changes
.BR CONVFMT
(which no historical program would do), it will receive the historical
behavior associated with internal string conversions.
.P
The POSIX
.IR awk
lexical and syntactic conventions are specified more formally than in
other sources. Again the intent has been to specify historical
practice. One convention that may not be obvious from the formal
grammar as in other verbal descriptions is where
<newline>
characters are acceptable. There are several obvious placements such as
terminating a statement, and a
<backslash>
can be used to escape
<newline>
characters between any lexical tokens. In addition,
<newline>
characters without
<backslash>
characters can follow a comma, an open brace, a logical AND operator (\c
.BR \(dq&&\(dq ),
a logical OR operator (\c
.BR \(dq||\(dq ),
the
.BR do
keyword, the
.BR else
keyword, and the closing parenthesis of an
.BR if ,
.BR for ,
or
.BR while
statement. For example:
.sp
.RS 4
.nf

{ print $1,
        $2 }
.fi
.P
.RE
.P
The requirement that
.IR awk
add a trailing
<newline>
to the program argument text is to simplify the grammar, making it
match a text file in form. There is no way for an application or test
suite to determine whether a literal
<newline>
is added or whether
.IR awk
simply acts as if it did.
.P
POSIX.1\(hy2008 requires several changes from historical implementations in order
to support internationalization. Probably the most subtle of these is
the use of the decimal-point character, defined by the
.IR LC_NUMERIC
category of the locale, in representations of floating-point numbers.
This locale-specific character is used in recognizing numeric input, in
converting between strings and numeric values, and in formatting
output. However, regardless of locale, the
<period>
character (the decimal-point character of the POSIX locale) is the
decimal-point character recognized in processing
.IR awk
programs (including assignments in command line arguments). This is
essentially the same convention as the one used in the ISO\ C standard. The
difference is that the C language includes the
\fIsetlocale\fR()
function, which permits an application to modify its locale. Because of
this capability, a C application begins executing with its locale set
to the C locale, and only executes in the environment-specified locale
after an explicit call to
\fIsetlocale\fR().
However, adding such an elaborate new feature to the
.IR awk
language was seen as inappropriate for POSIX.1\(hy2008. It is possible to execute
an
.IR awk
program explicitly in any desired locale by setting the environment in
the shell.
.P
The undefined behavior resulting from NULs in extended regular
expressions allows future extensions for the GNU
.IR gawk
program to process binary data.
.P
The behavior in the case of invalid
.IR awk
programs (including lexical, syntactic, and semantic errors) is
undefined because it was considered overly limiting on implementations
to specify. In most cases such errors can be expected to produce a
diagnostic and a non-zero exit status. However, some implementations
may choose to extend the language in ways that make use of certain
invalid constructs. Other invalid constructs might be deemed worthy of
a warning, but otherwise cause some reasonable behavior. Still other
constructs may be very difficult to detect in some implementations.
Also, different implementations might detect a given error during an
initial parsing of the program (before reading any input files) while
others might detect it when executing the program after reading some
input. Implementors should be aware that diagnosing errors as early as
possible and producing useful diagnostics can ease debugging of
applications, and thus make an implementation more usable.
.P
The unspecified behavior from using multi-character
.BR RS
values is to allow possible future extensions based on extended regular
expressions used for record separators. Historical implementations take
the first character of the string and ignore the others.
.P
Unspecified behavior when
.IR split (\c
.IR string ,\c
.IR array ,\c
<null>)
is used is to allow a proposed future extension that would split up a
string into an array of individual characters.
.P
In the context of the
.BR getline
function, equally good arguments for different precedences of the
.BR |
and
.BR <
operators can be made. Historical practice has been that:
.sp
.RS 4
.nf

getline < "a" "b"
.fi
.P
.RE
.P
is parsed as:
.sp
.RS 4
.nf

( getline < "a" ) "b"
.fi
.P
.RE
.P
although many would argue that the intent was that the file
.BR ab
should be read. However:
.sp
.RS 4
.nf

getline < "x" + 1
.fi
.P
.RE
.P
parses as:
.sp
.RS 4
.nf

getline < ( "x" + 1 )
.fi
.P
.RE
.P
Similar problems occur with the
.BR |
version of
.BR getline ,
particularly in combination with
.BR $ .
For example:
.sp
.RS 4
.nf

$"echo hi" | getline
.fi
.P
.RE
.P
(This situation is particularly problematic when used in a
.BR print
statement, where the
.BR |getline
part might be a redirection of the
.BR print .)
.P
Since in most cases such constructs are not (or at least should not) be
used (because they have a natural ambiguity for which there is no
conventional parsing), the meaning of these constructs has been made
explicitly unspecified. (The effect is that a conforming application that
runs into the problem must parenthesize to resolve the ambiguity.)
There appeared to be few if any actual uses of such constructs.
.P
Grammars can be written that would cause an error under these
circumstances. Where backwards-compatibility is not a large
consideration, implementors may wish to use such grammars.
.P
Some historical implementations have allowed some built-in functions to
be called without an argument list, the result being a default argument
list chosen in some ``reasonable'' way. Use of
.BR length
as a synonym for
.BR "length($0)"
is the only one of these forms that is thought to be widely known or
widely used; this particular form is documented in various places (for
example, most historical
.IR awk
reference pages, although not in the referenced \fIThe AWK Programming Language\fP) as legitimate practice.
With this exception, default argument lists have always been
undocumented and vaguely defined, and it is not at all clear how (or
if) they should be generalized to user-defined functions. They add no
useful functionality and preclude possible future extensions that might
need to name functions without calling them. Not standardizing them
seems the simplest course. The standard developers considered that
.BR length
merited special treatment, however, since it has been documented in the
past and sees possibly substantial use in historical programs.
Accordingly, this usage has been made legitimate, but Issue\ 5
removed the obsolescent marking for XSI-conforming implementations
and many otherwise conforming applications depend on this feature.
.P
In
.BR sub
and
.BR gsub ,
if
.IR repl
is a string literal (the lexical token
.BR STRING ),
then two consecutive
<backslash>
characters should be used in the string to ensure a single
<backslash>
will precede the
<ampersand>
when the resultant string is passed to the function. (For example,
to specify one literal
<ampersand>
in the replacement string, use
.BR gsub (\c
.BR ERE ,
.BR \(dq\e\e&\(dq ).)
.P
Historically, the only special character in the
.IR repl
argument of
.BR sub
and
.BR gsub
string functions was the
<ampersand>
(\c
.BR '&' )
character and preceding it with the
<backslash>
character was used to turn off its special meaning.
.P
The description in the ISO\ POSIX\(hy2:\|1993 standard introduced behavior such that the
<backslash>
character was another special character and it was unspecified whether
there were any other special characters. This description introduced
several portability problems, some of which are described below, and so
it has been replaced with the more historical description. Some of the
problems include:
.IP " *" 4
Historically, to create the replacement string, a script could use
.BR gsub (\c
.BR ERE ,
.BR \(dq\e\e&\(dq ),
but with the ISO\ POSIX\(hy2:\|1993 standard wording, it was necessary to use
.BR gsub (\c
.BR ERE ,
.BR \(dq\e\e\e\e&\(dq ).
The
<backslash>
characters are doubled here because all string literals are subject to
lexical analysis, which would reduce each pair of
<backslash>
characters to a single
<backslash>
before being passed to
.BR gsub .
.IP " *" 4
Since it was unspecified what the special characters were, for portable
scripts to guarantee that characters are printed literally, each
character had to be preceded with a
<backslash>.
(For example, a portable script had to use
.BR gsub (\c
.BR ERE ,
.BR \(dq\e\eh\e\ei\(dq )
to produce a replacement string of
.BR \(dqhi\(dq .)
.P
The description for comparisons in the ISO\ POSIX\(hy2:\|1993 standard did not properly describe
historical practice because of the way numeric strings are compared as
numbers. The current rules cause the following code:
.sp
.RS 4
.nf

if (0 == "000")
    print "strange, but true"
else
    print "not true"
.fi
.P
.RE
.P
to do a numeric comparison, causing the
.BR if
to succeed. It should be intuitively obvious that this is incorrect
behavior, and indeed, no historical implementation of
.IR awk
actually behaves this way.
.P
To fix this problem, the definition of
.IR "numeric string"
was enhanced to include only those values obtained from specific
circumstances (mostly external sources) where it is not possible to
determine unambiguously whether the value is intended to be a string or
a numeric.
.P
Variables that are assigned to a numeric string shall also be treated
as a numeric string. (For example, the notion of a numeric string can
be propagated across assignments.) In comparisons, all variables having
the uninitialized value are to be treated as a numeric operand
evaluating to the numeric value zero.
.P
Uninitialized variables include all types of variables including
scalars, array elements, and fields. The definition of an uninitialized
value in
.IR "Variables and Special Variables"
is necessary to describe the value placed on uninitialized variables
and on fields that are valid (for example,
.BR <
.BR $NF )
but have no characters in them and to describe how these variables are
to be used in comparisons. A valid field, such as
.BR $1 ,
that has no characters in it can be obtained from an input line of
.BR \(dq\et\et\(dq 
when
.BR FS= \c
.BR '\et' .
Historically, the comparison (\c
.BR $1< 10)
was done numerically after evaluating
.BR $1
to the value zero.
.P
The phrase ``.\|.\|. also shall have the numeric value of the numeric
string'' was removed from several sections of the ISO\ POSIX\(hy2:\|1993 standard because is
specifies an unnecessary implementation detail. It is not necessary for
POSIX.1\(hy2008 to specify that these objects be assigned two different values.
It is only necessary to specify that these objects may evaluate to two
different values depending on context.
.P
Historical implementations of
.IR awk
did not parse hexadecimal integer or floating constants like
.BR \(dq0xa\(dq 
and
.BR \(dq0xap0\(dq .
Due to an oversight, the 2001 through 2004 editions of this standard
required support for hexadecimal floating constants. This was due to
the reference to
\fIatof\fR().
This version of the standard allows but does not require implementations
to use
\fIatof\fR()
and includes a description of how floating-point numbers are recognized
as an alternative to match historic behavior. The intent of this change
is to allow implementations to recognize floating-point constants
according to either the ISO/IEC\ 9899:\|1990 standard or ISO/IEC\ 9899:\|1999 standard, and to allow (but not require)
implementations to recognize hexadecimal integer constants.
.P
Historical implementations of
.IR awk
did not support floating-point infinities and NaNs in
.IR "numeric strings" ;
e.g.,
.BR \(dq-INF\(dq 
and
.BR \(dqNaN\(dq .
However, implementations that use the
\fIatof\fR()
or
\fIstrtod\fR()
functions to do the conversion picked up support for these values if they
used a ISO/IEC\ 9899:\|1999 standard version of the function instead of a ISO/IEC\ 9899:\|1990 standard version. Due to
an oversight, the 2001 through 2004 editions of this standard did not
allow support for infinities and NaNs, but in this revision support is
allowed (but not required). This is a silent change to the behavior of
.IR awk
programs; for example, in the POSIX locale the expression:
.sp
.RS 4
.nf

("-INF" + 0 < 0)
.fi
.P
.RE
.P
formerly had the value 0 because
.BR \(dq-INF\(dq 
converted to 0, but now it may have the value 0 or 1.
.SH "FUTURE DIRECTIONS"
A future version of this standard may require the
.BR \(dq!=\(dq 
and
.BR \(dq==\(dq 
operators to perform string comparisons by checking if the strings are
identical (and not by checking if they collate equally).
.SH "SEE ALSO"
.IR "Section 1.3" ", " "Grammar Conventions",
.IR "\fIgrep\fR\^",
.IR "\fIlex\fR\^",
.IR "\fIsed\fR\^"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 5" ", " "File Format Notation",
.IR "Section 6.1" ", " "Portable Character Set",
.IR "Chapter 8" ", " "Environment Variables",
.IR "Chapter 9" ", " "Regular Expressions",
.IR "Section 12.2" ", " "Utility Syntax Guidelines"
.P
The System Interfaces volume of POSIX.1\(hy2017,
.IR "\fIatof\fR\^(\|)",
.IR "\fIexec\fR\^",
.IR "\fIisspace\fR\^(\|)",
.IR "\fIpopen\fR\^(\|)",
.IR "\fIsetlocale\fR\^(\|)",
.IR "\fIstrtod\fR\^(\|)"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
